Image forming optical system and optical device using the same

ABSTRACT

An image forming optical system includes, comprises, in order from the object side toward the image side, a first lens unit with positive refracting power, a second lens unit with positive refracting power, a third lens unit with negative refracting power, a fourth lens unit with positive refracting power, and an aperture stop interposed between the third lens unit and the fourth lens unit. The image forming optical system has a variable magnification optical system in which spacings between these lens units are changed to vary the imaging magnification, changes the imaging magnification while constantly keeping the object-to-image distance of the image forming optical system, and in at least one variable magnification state where the imaging magnification is changed, satisfies the following conditions: 
     
       
         | En|/L   &gt;0.4   
       
     
     
       
         | Ex|/|L   /β|&gt;0.4   
       
     
     where En is a distance from a first lens surface on the object side of the variable magnification optical system to the entrance pupil of the image forming optical system, L is the object-to-image distance of the image forming optical system, Ex is a distance from the last lens surface on the image side of the variable magnification optical system to the exit pupil of the image forming optical system, and β is the magnification of the whole of the image forming optical system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable magnification lens in whichimaging magnification can be changed in accordance with a photographingpurpose, and to an optical system in which an image recorded on a filmcan be photographed at magnification most suitable for the film and anoptical device, such as an image transforming device, using this opticalsystem.

2. Description of Related Art

Image forming optical systems which are bilateral telecentric and arecapable of changing the imaging magnification are proposed, for example,by Japanese Patent Kokai No. 2001-27726 and Japanese Patent No.2731481.

The optical system proposed by Kokai No. 2001-27726 includes, in orderfrom the object side, a first lens unit with positive refracting power,a second lens unit with positive refracting power, a third lens unitwith negative refracting power, and a fourth lens unit with positiverefracting power. It is constructed as an optical system which isbilateral telecentric and is capable of changing the imagingmagnification.

In this optical system, however, when the imaging magnification ischanged, an object-to-image distance is varied, and thus there is theneed to move the entire optical system in accordance with a change ofthe magnification.

The optical system proposed by Patent No. 2731481 includes, in orderfrom the object side, a first lens unit with positive refracting power,a second lens unit with negative refracting power, and a third lens unitwith positive refracting power. It is constructed as an optical systemwhich is bilateral telecentric and changes the imaging magnificationwhile constantly keeping the object-to-image distance.

In this optical system, however, its F-number fluctuates considerably,depending on the imaging magnification, for example, so that when theimaging magnification is 0.25×, an image-side F-number is 8.741 and whenthe imaging magnification is 1.00×, the image-side F-number is 14.286.Therefore, the problem arises that when the imaging magnification ischanged, the brightness of a camera must be adjusted accordingly.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide an imageforming optical system in which even when the imaging magnification ischanged, the object-to-image distance remains unchanged and there islittle fluctuation in F-number.

In order to accomplish this object, the image forming optical systemaccording to the present invention includes, in order from the objectside toward the image side, a first lens unit with positive refractingpower, a second lens unit with positive refracting power, a third lensunit with negative refracting power, a fourth lens unit with positiverefracting power, and an aperture stop interposed between the third lensunit and the fourth lens unit. The image forming optical system has avariable magnification optical system in which spacings between thefirst lens unit and the second lens unit, between the second lens unitand the third lens unit, and between the third lens unit and the fourthlens unit are changed to vary the imaging magnification. In this case,the image forming optical system changes the imaging magnification whileconstantly keeping the object-to-image distance thereof, and in at leastone variable magnification state where the imaging magnification ischanged, satisfies the following conditions:

|En|/L>0.4

|Ex|/|L/β|>0.4

where En is a distance from a first lens surface on the object side ofthe variable magnification optical system to the entrance pupil of theimage forming optical system, L is the object-to-image distance of theimage forming optical system, Ex is a distance from the last lenssurface on the image side of the variable magnification optical systemto the exit pupil of the image forming optical system, and β is themagnification of the whole of the image forming optical system.

The image forming optical system of the present invention also satisfiesthe following conditions:

1.0<MAXFNO<8.0

|ΔFNO/Δβ|<5

where MAXFNO is an object-side F-number which is smallest when theimaging magnification of the image forming optical system is changed,ΔFNO is a difference between the object-side F-number at the minimummagnification of the whole of the image forming optical system and thatat the maximum magnification of the whole of the image forming opticalsystem, and Δβ is a difference between the minimum magnification of thewhole of the image forming optical system and the maximum magnificationof the whole of the image forming optical system.

The image forming optical system of the present invention furthersatisfies the following condition:

0.6<|(R3F+R3b)/(R3f−R3b)|<5.0

where R3f is the radius of curvature of the most object-side surface ofthe third lens unit and R3b is the radius of curvature of the mostimage-side surface of the third lens unit.

The optical device of the present invention uses the image formingoptical system of the present invention.

According to the present invention, the image forming optical system inwhich even when the imaging magnification is changed, theobject-to-image distance remains unchanged and there is littlefluctuation in F-number, and the optical device using this image formingoptical system, can be obtained.

This and other objects as well as the features and advantages of thepresent invention will become apparent from the following detaileddescription when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.3×in a first embodiment of the image forming optical system according tothe present invention;

FIG. 1B is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.4×in the first embodiment;

FIG. 1C is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.5×in the first embodiment;

FIGS. 2A, 2B, and 2C are diagrams showing aberration characteristics infocusing of an infinite object point where the imaging magnification isset to 0.4× in the first embodiment;

FIG. 3A is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.3×in a second embodiment of the image forming optical system according tothe present invention;

FIG. 3B is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.4×in the second embodiment;

FIG. 3C is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.5×in the second embodiment;

FIGS. 4A, 4B, and 4C are diagrams showing aberration characteristics infocusing of an infinite object point where the imaging magnification isset to 0.4× in the second embodiment;

FIG. 5A is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.3×in a third embodiment of the image forming optical system according tothe present invention;

FIG. 5B is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.4×in the third embodiment;

FIG. 5C is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.5×in the third embodiment;

FIGS. 6A, 6B, and 6C are diagrams showing aberration characteristics infocusing of an infinite object point where the imaging magnification isset to 0.4× in the third embodiment;

FIG. 7A is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.3×in a fourth embodiment of the image forming optical system according tothe present invention;

FIG. 7B is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.4×in the fourth embodiment;

FIG. 7C is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.5×in the fourth embodiment;

FIGS. 8A, 8B, and 8C are diagrams showing aberration characteristics infocusing of an infinite object point where the imaging magnification isset to 0.4× in the fourth embodiment;

FIG. 9A is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.3×in a fifth embodiment of the image forming optical system according tothe present invention;

FIG. 9B is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.4×in the fifth embodiment;

FIG. 9C is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.5×in the fifth embodiment;

FIGS. 10A, 10B, and 10C are diagrams showing aberration characteristicsin focusing of an infinite object point where the imaging magnificationis set to 0.4× in the fifth embodiment;

FIG. 11A is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.3×in a sixth embodiment of the image forming optical system according tothe present invention;

FIG. 11B is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.4×in the sixth embodiment;

FIG. 11C is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.5×in the sixth embodiment;

FIGS. 12A, 12B, and 12C are diagrams showing aberration characteristicsin focusing of an infinite object point where the imaging magnificationis set to 0.4× in the sixth embodiment;

FIG. 13A is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.3×in a seventh embodiment of the image forming optical system according tothe present invention;

FIG. 13B is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.4×in the seventh embodiment;

FIG. 13C is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.5×in the seventh embodiment;

FIGS. 14A, 14B, and 14C are diagrams showing aberration characteristicsin focusing of an infinite object point where the imaging magnificationis set to 0.4× in the seventh embodiment;

FIG. 15A is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.3×in an eighth embodiment of the image forming optical system according tothe present invention;

FIG. 15B is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.4×in the eighth embodiment;

FIG. 15C is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.5×in the eighth embodiment;

FIGS. 16A, 16B, and 16C are diagrams showing aberration characteristicsin focusing of an infinite object point where the imaging magnificationis set to 0.4× in the eighth embodiment;

FIG. 17A is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.3×in a ninth embodiment of the image forming optical system according tothe present invention;

FIG. 17B is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.4×in the ninth embodiment;

FIG. 17C is a sectional view showing an optical arrangement, developedalong the optical axis, where the imaging magnification is set to 0.5×in the ninth embodiment;

FIGS. 18A, 18B, and 18C are diagrams showing aberration characteristicsin focusing of an infinite object point where the imaging magnificationis set to 0.4× in the ninth embodiment;

FIG. 19 is a conceptual view showing an example of a telecine deviceusing the image forming optical system of the present invention; and

FIG. 20 is a view schematically showing an example of a height measuringdevice using the image forming optical system of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the description of the embodiments, reference is made to thefunction and effect of the present invention.

In the image forming optical system of the present invention, asmentioned above, the variable magnification optical system includes fourlens units with positive, positive, negative, and positive powers.Object-side lens units situated ahead of (on the object side of) thestop are the first lens unit with positive refracting power, the secondlens unit with positive refracting power, and the third lens unit withnegative refracting power, and the whole of these lens units isconstructed as a lens system with positive refracting power. The fourthlens unit situated behind (on the image side of) the stop is constructedas a lens system with positive refracting power. The aperture stop isinterposed between the third lens unit and the fourth lens unit.

The image forming optical system of the present invention is designed tochange the imaging magnification while constantly keeping theobject-to-image distance. That is, the image forming optical system ofthe present invention is such that a conjugate length is fixed.

The image forming optical system of the present invention satisfies thefollowing conditions in at least one variable magnification state wherethe imaging magnification is changed and is constructed to be bilateraltelecentric:

|En|/L>0.4  (1)

|Ex|/|L/β|>0.4  (2)

where En is a distance from a first lens surface on the object side ofthe variable magnification optical system to the entrance pupil of theimage forming optical system, L is the object-to-image distance of theimage forming optical system, Ex is a distance from the last lenssurface on the image side of the variable magnification optical systemto the exit pupil of the image forming optical system, and β is themagnification of the whole of the image forming optical system.

The image forming optical system of the present invention is constructedso that the stop is placed at the focal position of the lens systemcomposed of the first to third lens units situated on the object side ofthe stop. By this construction, the entrance pupil which is the image ofthe stop is projected at infinity. As a result, the image formingoptical system of the present invention is constructed as an object-sidetelecentric optical system.

The image forming optical system of the present invention is constructedso that the stop is placed at the focal position of the lens systemcomposed of the fourth lens unit situated on the image side of the stop.By this construction, the exit pupil which is the image of the stop isprojected at infinity. As a result, the image forming optical system ofthe present invention is also constructed as an image-side telecentricoptical system.

In the image forming optical system of the present invention constructedas mentioned above, the roles of multi-variators are assigned to boththe second lens unit with positive refracting power and the third lensunit with negative refracting power. By doing so, a combined focallength of the first to third lens units situated on the object side ofthe stop can be changed.

The image forming optical system of the present invention is constructedso that the stop is interposed between the third lens unit with negativerefracting power and the fourth lens unit with positive refractingpower. A variable magnification function is not imparted to the fourthlens unit located on the image side of the stop. The stop is designed sothat even when the imaging magnification is changed, the shift of theposition of the stop is suppressed as far as possible and is practicallyfixed. Thus, the stop is always located in the proximity of the focalposition of the fourth lens unit, and thereby the imaging magnificationcan be changed while maintaining an exit-side telecentric characteristicand F-number.

However, in order to maintain an object-side telecentric characteristicand fix the conjugate length while constantly keeping the F-number whenthe imaging magnification is changed, it is necessary to satisfy thefollowing conditions.

First, even when the magnification is changed, the stop must be locatedat the combined focal point of the first to third lens units on theobject side of the stop.

Second, even when the magnification is changed, a distance from thesurface of the object to that of the stop must be kept to be nearlyconstant.

In the construction of positive, negative, and positive powers, if thefirst lens unit is divided into two lens units with positive andnegative refracting powers, the balance between the refracting powerswill be destroyed. Consequently, chromatic aberration of magnificationand distortion are increased.

However, when the first lens unit is divided into two lens units withpositive and positive refracting powers, as in the present invention, sothat four lens units with positive, positive, negative, and positiverefracting powers are constructed, the amount of production ofaberration can be minimized.

In a bilateral telecentric optical system, even when the magnificationis changed, an off-axis ray at the position of stop is nearly parallelwith the optical axis. The lens unit located on the image side of thestop is the fourth lens unit alone, and since the fourth lens unit isnot moved, the focal length becomes constant. Therefore, when themagnification is changed, there is little fluctuation in F-number, andthus even when the magnification is changed, the brightness of thecamera need not be adjusted.

The construction of the object-side telecentric optical system like theimage forming optical system of the present invention offers advantagesdescribed below.

For example, the advantages are described with respect to a telecinedevice (a motion picture film scanner). The telecine device is such thata motion picture film is digitized. The telecine device is constructedso that the film is illuminated by an illumination optical system and animage is formed by a solid-state image sensor, such as a CCD, through animage forming optical system.

If the image forming optical system of the telecine device, like theimage forming optical system of the present invention, is constructed asthe object-side telecentric optical system, pupil matching between anillumination system and an image forming system will be facilitated andthe loss of the amount of light can be reduced. Furthermore, a change ofmagnification on the image plane caused by the disturbance of filmflatness can be minimized.

The construction of the image-side telecentric optical system like theimage forming optical system of the present invention offers advantagesdescribed below.

For example, the advantages are described with respect to a so-calledmulti-sensor camera which uses image sensors in accordance with colorssuch as R, B, and G. In this multi-sensor camera, a color dispersionprism is generally used. This prism is provided with a dispersioninterference film splitting light according to wavelength, namely adichroic film, deposited on its interface. If the exit pupil is locatedclose to the image plane, the angle of incidence at which a chief ray isincident on the interference film will be changed in accordance with theposition of an image point of the image plane. As a result, the opticalpath length of film thickness is changed and a color dispersioncharacteristic varies with the field angle. Thus, color reproducibilityis varied, that is, color shading is produced.

However, when the image forming optical system of the multi-sensorcamera, like the image forming optical system of the present invention,is constructed as the image-side telecentric optical system, the colorshading can be suppressed.

Here, for example, it is assumed that a solid-state image sensor, suchas a CCD, is placed on the image side of the color dispersion prism. Ifthe exit pupil is located close to the image plane, the chief ray willbe obliquely incident on a pixel. Hence, off-axis incident light ismainly blocked by a structure such as the CCD, and the amount of lightis impaired or light other than that to enter an originallight-receiving section is incident thereon. Consequently, signals otherthan original information are output. That is, shading occurs.

However, when the image forming optical system of the multi-sensorcamera, like the image forming optical system of the present invention,is constructed as the image-side telecentric optical system, the shadingcan be suppressed.

The image forming optical system of the present invention is alsoconstructed as a bilateral telecentric optical system. The imagingmagnification can thus be practically determined by the ratio betweenthe focal length of the lens units on the object side of the stop andthat of the lens unit on the image side of the stop.

Spacings between individual lens units located on the object side of thestop are changed to vary the focal length thereof. By doing so, theimaging magnification can be altered.

In the image forming optical system of the present invention, the firstlens unit has positive refracting power so that the entrance pupil whichis the image of the stop is projected at infinity. In doing so, thechief ray on the object side of the first lens unit is refractedparallel to the optical axis, and thereby the object-side telecentricoptical system can be realized.

In the image forming optical system of the present invention, the secondlens unit has positive refracting power and the third lens unit hasnegative refracting power. The spacing between the second lens unit andthe third lens unit is changed to vary a combined focal length of thesecond and third lens units. That is, the second and third lens unitsare designed to function as multi-variators. Thus, the second and thirdlens units are moved and thereby the magnification can be optimallyadjusted to the size of the object.

When the third lens unit, as in the image forming optical system of thepresent invention, is constructed to have negative refracting power, thePetzval sum is increased, and an optical system that is free ofcurvature of field can be obtained.

In the image forming optical system of the present invention, thepositive refracting power is imparted to the fourth lens unit so thatthe exit pupil which is the image of the stop is projected at infinity.In doing so, the chief ray on the image side of the fourth lens unit ismade parallel to the optical axis, and thereby the image-sidetelecentric optical system can be realized.

When the image forming optical system of the present invention providedwith a variable magnification function described above is used toconstitute the optical device, there are advantages described below.

For example, the advantages are explained with respect to the telecinedevice as mentioned above. The telecine device is such that a videocamera is attached to a film imaging device. It is constructed so that afilm image is converted into a video signal and is digitized.

On the other hand, the motion picture film has a plurality of standardsand the size of a film image section varies with each standard. Forexample, the size of a standard 35 mm film is 16×21.9 mm and a Europeanwide film measures 11.9×21.95 mm. In this way, aspect ratios vary withfilm standards. The size of the imaging plane of the CCD, for example,in a 2/3 type CCD solid-state image sensor, is 5.4×9.6 mm. In order tophotograph an image with high-precision and -density pixels, it isdesirable to acquire image information over the entire CCD imaging area.For this, it becomes necessary to change the imaging magnification tothe film standard.

However, when the image forming optical system of the present inventionis used to constitute the optical device, films of various standards canbe digitized, for example, in the telecine device. In this case, evenwhen the imaging magnification is changed, the conjugate length remainsunchanged and the image-side F-number can be maintained with littlefluctuation.

For example, when the image forming optical system of the presentinvention is used in the multi-sensor camera, color shading by the colordispersion prism and the shading of the CCD camera can be suppressed.Moreover, the imaging magnification can be changed, without moving thecamera, in accordance with the film standard and the size of the object,and even when the magnification is changed, there is no need to adjustbrightness.

In the image forming optical system of the present invention, to obtainfurther bilateral telecentricity, it is desirable that when the imagingmagnification is changed, the optical system, instead of satisfyingConditions (1) and (2) in at least one variable magnification state,satisfies the following conditions:

|En|/L>0.8  (1′)

|Ex|/|L/β|<0.8  (2′)

It is more desirable to satisfy the following conditions:

|En|/L<1.6  (1″)

|Ex|/|L/β|<1.6  (2″)

In the image forming optical system of the present invention, theF-number is defined by the following conditions:

1.0<MAXFNO<8.0  (3)

|ΔFNO/Δβ|<5  (4)

where MAXFNO is an object-side F-number which is smallest when theimaging magnification of the image forming optical system is changed,ΔFNO is a difference between the object-side F-number at the minimummagnification of the whole of the image forming optical system and thatat the maximum magnification of the whole of the image forming opticalsystem, and Δβ is a difference between the minimum magnification of thewhole of the image forming optical system and the maximum magnificationof the whole of the image forming optical system.

If the F-number is extremely small, the number of lenses must beincreased to correct aberration. As a result, the problem arises thatthe entire length of the optical system is increased. On the other hand,if the F-number is extremely large, the amount of light becomesinsufficient, which is not suitable for motion picture photography.

However, when the optical system satisfies Condition (3), the F-numberis neither extremely small nor large. Hence, the above problem, such asan increase of the entire length of the optical system or unsuitabilityfor motion picture photography, can be solved. Also, the F-number standsfor the brightness of an optical system, and as its numerical value isdecreased, the optical system becomes bright.

If the value of |ΔFNO/Δβ| is extremely large, the fluctuation of theimage-side F-number where the magnification is changed becomesprominent. As a result, the brightness of the camera must be adjusted.

However, when the optical system satisfies Condition (4), there is noneed to adjust the brightness of the camera.

Preferably, it is desirable to satisfy the following conditions:

2.0<MAXFNO<5.6  (3′)

|ΔFNO/Δβ|<3  (4′)

It is more desirable to satisfy the following conditions:

3.0<MAXFNO<4.0  (3″)

|ΔFNO/Δβ|<1  (4″)

In the image forming optical system of the present invention, it isdesirable that the most object-side lens of the first lens unit haspositive refracting power.

When the most object-side lens of the first lens unit is constructed asa positive lens, the height of an off-axis beam can be lowered, and thusaberration is minimized.

In the image forming optical system of the present invention, it isdesirable that the first lens unit is constructed with, in order fromthe object side, positive, negative, and positive lenses.

When the first lens unit is constructed in this way, chromaticaberration of magnification and off-axis chromatic aberration can becorrected.

In the image forming optical system of the present invention, it isdesirable to satisfy the following condition:

0.6<|(R3f+R3b)/(R3f−R3b)|<5.0  (5)

where |(R3f+R3b)/(R3f−R3b)| is a virtual shape factor, R3f is the radiusof curvature of the most object-side surface of the third lens unit, andR3b is the radius of curvature of the most image-side surface of thethird lens unit.

When the optical system satisfies this condition, the fluctuation ofoff-axis aberration can be kept to a minimum even when the third lensunit is moved along the optical axis to change the magnification.

If the value of the virtual shape factor exceeds the upper limit, thecurvature of the most object-side surface of the third lens unit willapproach that of the most image-side surface of the third lens unit.Thus, the refracting power of the third lens unit is extremely weakened.Consequently, when the magnification is changed, a considerable amountof movement of the third lens unit is required. If the amount ofmovement of the third lens unit is large, a ray height at which theoff-axis beam is incident on the third lens unit will fluctuate. As aresult, the fluctuation of off-axis aberration becomes prominent.

On the other hand, if the value of the virtual shape factor is below thelower limit, the refracting power of the third lens unit will beextremely strengthened. Consequently, the angle of incidence of theoff-axis beam on the third lens unit is increased, and the fluctuationof off-axis aberration caused by the movement of the third lens unitbecomes pronounced.

However, when the optical system satisfies Condition (5), it isavoidable that the refracting power of the third lens unit is extremelystrengthened or weakened, and the problem that the fluctuation ofoff-axis aberration becomes pronounced, as mentioned above, can besolved.

Preferably, it is desirable to satisfy the following condition:

1.2<|(R3f+R3b)/(R3f−R3b)|<3.5  (5′)

It is more desirable to satisfy the following condition:

2.0<|(R3f+R3b)/(R3f−R3b)|<3.0  (5″)

In the image forming optical system of the present invention, it isdesirable that the third lens unit has at least two meniscus lenses,each with a convex surface directed toward the object side. It is moredesirable to have at least three meniscus lenses.

More specifically, for example, it is favorable that the third lens unithas two negative meniscus lenses, each with a convex surface directedtoward the object side, and a positive meniscus lens with a convexsurface directed toward the object side.

Since the third lens unit is located close to the stop, off-axis raysare incident on the third lens unit at almost the same angle,irrespective of the field angles.

A meniscus lens whose convex surface is directed toward the object side,that is, whose object-side surface has positive refracting power,practically has the minimum deflection angle with respect to axial andoff-axis beams of individual field angles, and hence the production ofaberration can be prevented.

In accordance with the drawings, the embodiments of the presentinvention will be described below.

First Embodiment

FIGS. 1A, 1B, and 1C show optical arrangements where imagingmagnifications are set to 0.3×, 0.4×, and 0.5×, respectively, in thefirst embodiment. FIGS. 2A, 2B, and 2C show aberration characteristicsin focusing of an infinite object point where the imaging magnificationis set to 0.4× in the first embodiment.

The image forming optical system of the first embodiment includes avariable magnification optical system Z. In this figure, referencesymbol P represents a prism, CG represents a glass cover, and Irepresents an imaging plane.

The variable magnification optical system Z comprises, in order form theobject side toward the image side, a first lens unit G1 with positiverefracting power, a second lens unit G2 with positive refracting power,a third lens unit G3 with negative refracting power, an aperture stop S,and a fourth lens unit G4 with positive refracting power.

The first lens unit G1 includes a biconvex lens L1 ₁, a biconcave lensL1 ₂, and a biconvex lens L1 ₃, arranged in this order from the objectside.

The second lens unit G2 includes a negative meniscus lens L2 ₁ with aconvex surface directed toward the object side, a biconvex lens L2 ₂, anegative meniscus lens L2 ₃ with a concave surface directed toward theobject side, and a biconvex lens L2 ₄, arranged in this to order fromthe object side.

The third lens unit G3 includes a positive meniscus lens L3 ₁ with aconvex surface directed toward the object side, a negative meniscus lensL3 ₂ with a convex surface directed toward the object side, and anegative meniscus lens L3 ₃ with a convex surface directed toward theobject side.

The fourth lens unit G4 includes a cemented lens with a biconcave lensL4 ₁ and a biconvex lens L4 ₂, a biconcave lens L4 ₃, a biconvex lens L4₄, a biconvex lens L4 ₅, and a biconvex lens L4 ₆.

When the magnification is changed from 0.3× to 0.5× in focusing of theinfinite object point, the first lens unit G1, after being moved oncetoward the object side, is moved toward the image side; the second lensunit G2 is moved toward the object side; the third lens unit G3 ismoved, together with the stop S, toward the image side; and the fourthlens unit G4 is moved toward the image side so that spacing between thethird lens unit G3 and the fourth lens unit G4 is slightly widened.

The object-to-image distance where the magnification is changed isconstantly maintained.

Subsequently, numerical data of optical members constituting the imageforming optical system of the first embodiment are listed below. In thenumerical data, r₀, r₁, r₂, . . . denote radii of curvature of surfacesof individual optical members, shown in this order from the object side;d₀, d₁, d₂, . . . denote thicknesses of individual optical members orspacings between them (unit: mm), shown in this order from the objectside; n_(e1), n_(e2), . . . denote refractive indices of individualoptical members at the e line, shown in this order from the object side;and v_(e1), v_(e2), . . . denote Abbe's numbers of individual opticalmembers at the e line, shown in this order from the object side. Thesesymbols are also applied to the numerical data of other embodiments.

Numerical data 1 Image height: 5.783 r₀ = ∞ (object) d₀ = 50.000 r₁ = ∞(object surface) d₁ = D1 r₂ = 189.5313 d₂ = 7.308 n_(e2) = 1.48915ν_(e2) = 70.04 r₃ −117.0877 d₃ = 10.588 r₄ = −6124.8097 d₄ = 6.910n_(e4) = 1.61639 ν_(e4) = 44.15 r₅ = 67.5133 d₅ = 12.028 r₆ = 88.2299 d₆= 8.685 n_(e6) = 1.43985 ν_(e6) = 94.53 r₇ = −425.3119 d₇ = D7 r₈ =148.1127 d₈ = 6.000 n_(e8) = 1.61639 ν_(e8) = 44.15 r₉ = 64.7754 d₉ =5.355 r₁₀ = 88.2208 d₁₀ = 8.016 n_(e10) = 1.43985 ν_(e10) = 94.53 r₁₁ =−81.9368 d₁₁ = 1.062 r₁₂ = −69.6148 d₁₂ = 7.000 n_(e12) = 1.61639ν_(e12) = 44.15 r₁₃ = −171.6506 d₁₃ = 17.627 r₁₄ = 210.1703 d₁₄ = 6.814n_(e14) = 1.43985 ν_(e14) = 94.53 r₁₅ = −82.3361 d₁₅ = D15 r₁₆ = 40.6305d₁₆ = 4.323 n_(e16) = 1.69417 ν_(e16) = 30.83 r₁₇ = 250.0598 d₁₇ = 0.300r₁₈ = 25.0517 d₁₈ = 9.360 n_(e18) = 1.72538 ν_(e18) = 34.47 r₁₉ =21.5375 d₁₉ = 1.156 r₂₀ = 41.2143 d₂₀ = 2.000 n_(e20) = 1.72538 ν_(e20)= 34.47 r₂₁ = 15.8016 d₂₁ = 2.560 r₂₂ = ∞ (aperture stop) d₂₂ = D22 r₂₃= −29.2488 d₂₃ = 2.000 n_(e23) = 1.61669 ν_(e23) = 44.02 r₂₄ = 23.4936d₂₄ = 7.647 n_(e24) = 1.48915 ν_(e24) = 70.04 r₂₅ = −17.8845 d₂₅ = 3.043r₂₆ = −13.7038 d₂₆ = 1.417 n_(e26) = 1.61639 ν_(e26) = 44.15 r₂₇ =89.8893 d₂₇ = 4.829 r₂₈ = 707.1568 d₂₈ = 8.564 n_(e28) = 1.43985 ν_(e28)= 94.53 r₂₉ = −18.1649 d₂₉ = 0.325 r₃₀ = 69.4722 d₃₀ = 5.111 n_(e30) =1.43985 ν_(e30) = 94.53 r₃₁ = −90.8646 d₃₁ = 0.300 r₃₂ = 62.9985 d₃₂ =4.778 n_(e32) = 1.43985 ν_(e32) = 94.53 r₃₃ = −179.4454 d₃₃ = D33 r₃₄ =∞ d₃₄ = 33.000 n_(e34) = 1.61173 ν_(e34) = 46.30 r₃₅ = ∞ d₃₅ = 13.200n_(e35) = 1.51825 ν_(e35) = 63.93 r₃₆ = ∞ d₃₆ = 0.500 r₃₇ = ∞ (imagingplane) d₃₇ = 0.000 Zoom data 0.3× 0.4× 0.5× D1 39.880 37.812 44.358 D7109.204 77.238 48.939 D15 3.000 37.903 60.723 D22 3.552 4.754 6.263 D3321.051 18.980 16.405 Condition parameters and others Magnification: β0.3× 0.4× 0.5× Entrance pupil position: En 1160.856 20252.775 −1133.552Object-to-image distance: L 428.492 428.492 428.492 |EN|/L 2.709 47.2652.645 Exit pupil position: Ex −352.468 −578.834 −1818.976 |Ex|/|L/β|0.247 0.540 2.123 F-number: FNO 3.500 3.536 3.598 The amount offluctuation of FNO 0.098 ΔFNO/Δβ 0.490 Radius of curvature on the objectside: R3f 40.630 Radius of curvature on the image side: R3b 15.802|(R3f + R3b)/(R3f − R3b) 2.273

Second Embodiment

FIGS. 3A, 3B, and 3C show optical arrangements where imagingmagnifications are set to 0.3×, 0.4×, and 0.5×, respectively, in thesecond embodiment. FIGS. 4A, 4B, and 4C show aberration characteristicsin focusing of an infinite object point where the imaging magnificationis set to 0.4× in the second embodiment.

The image forming optical system of the second embodiment includes thevariable magnification optical system Z. In this figure, again,reference symbol P represents a prism, CG represents a glass cover, andI represents an imaging plane.

The variable magnification optical system Z comprises, in order form theobject side toward the image side, the first lens unit G1 with positiverefracting power, the second lens unit G2 with positive refractingpower, the third lens unit G3 with negative refracting power, theaperture stop S, and the fourth lens unit G4 with positive refractingpower.

The first lens unit G1 includes the biconvex lens L1 ₁, a negativemeniscus lens L1 ₂′ with a convex surface directed toward the objectside, and the biconvex lens L1 ₃, arranged in this order from the objectside.

The second lens unit G2 includes the negative meniscus lens L2 ₁ with aconvex surface directed toward the object side, the biconvex lens L2 ₂,the negative meniscus lens L2 ₃ with a concave surface directed towardthe object side, and the biconvex lens L2 ₄, arranged in this order fromthe object side.

The third lens unit G3 includes the positive meniscus lens L3 ₁ with aconvex surface directed toward the object side, the negative meniscuslens L3 ₂ with a convex surface directed toward the object side, and thenegative meniscus lens L3 ₃ with a convex surface directed toward theobject side, arranged in this order from the object side.

The fourth lens unit G4 includes the cemented lens with the biconcavelens L4 ₁ and the biconvex lens L4 ₂, the biconcave lens L4 ₃, thebiconvex lens L4 ₄, the biconvex lens L4 ₅, and the biconvex lens L4 ₆,arranged in this order from the object side.

When the magnification is changed from 0.3× to 0.5× in focusing of theinfinite object point, the first lens unit G1, after being moved oncetoward the object side, is moved toward the image side; the second lensunit G2 is moved toward the object side; the third lens unit G3 remainsfixed together with the stop S; and the fourth lens unit G4 is movedtoward the image side so that the spacing between the third lens unit G3and the fourth lens unit G4 is slightly widened.

The object-to-image distance where the magnification is changed isconstantly maintained.

Subsequently, numerical data of optical members constituting the imageforming optical system of the second embodiment are listed below.

Numerical data 2 Image height: 5.783 r₀ = ∞ (object) d₀ = 50.000 r₁ = ∞(object surface) d₁ = D1 r₂ = 172.4277 d₂ = 6.648 n_(e2) = 1.48915ν_(e2) = 70.04 r₃ = −112.2625 d₃ = 7.313 r₄ = 1492.6672 d₄ = 7.985n_(e4) = 1.61639 ν_(e4) = 44.15 r₅ = 62.4069 d₅ = 12.125 r₆ = 79.8565 d₆= 9.415 n_(e6) = 1.43985 ν_(e6) = 94.53 r₇ = −1585.7009 d₇ = D7 r₈ =151.8708 d₈ = 6.000 n_(e8) = 1.61639 ν_(e8) = 44.15 r₉ = 64.4718 d₉ =5.384 r₁₀ = 86.7203 d₁₀ = 8.163 n_(e10) = 1.43985 ν_(e10) = 94.53 r₁₁ =−80.8037 d₁₁ = 1.049 r₁₂ = −68.7719 d₁₂ = 6.410 n_(e12) = 1.61639ν_(e12) = 44.15 r₁₃ = −178.7270 d₁₃ = 16.603 r₁₄ = 219.0646 d₁₄ = 6.722n_(e14) = 1.43985 ν_(e14) = 94.53 r₁₅ = −81.1984 d₁₅ = D15 r₁₆ = 40.1465d₁₆ = 4.375 n_(e16) = 1.69417 ν_(e16) = 30.83 r₁₇ = 229.4681 d₁₇ = 0.300r₁₈ = 24.8118 d₁₈ = 9.366 n_(e18) = 1.72538 ν_(e18) = 34.47 r₁₉ =21.1952 d₁₉ = 1.169 r₂₀ = 40.9998 d₂₀ = 2.000 n_(e20) = 1.72538 ν_(e20)= 34.47 r₂₁ = 15.9793 d₂₁ = 2.555 r₂₂ = ∞ (aperture stop) d₂₂ = D22 r₂₃= −29.1565 d₂₃ = 2.000 n_(e23) = 1.61669 ν_(e23) = 44.02 r₂₄ = 23.6864d₂₄ = 7.373 n_(e24) = 1.48915 ν_(e24) = 70.04 r₂₅ = −18.0561 d₂₅ = 3.435r₂₆ = −13.7966 d₂₆ = 1.355 n_(e26) = 1.61639 ν_(e26) = 44.15 r₂₇ =84.7189 d₂₇ = 4.778 r₂₈ = 547.3608 d₂₈ = 8.544 n_(e28) = 1.43985 ν_(e28)= 94.53 r₂₉ = −18.0837 d₂₉ = 0.300 r₃₀ = 70.0296 d₃₀ = 5.063 n_(e30) =1.43985 ν_(e30) = 94.53 r₃₁ = −93.9274 d₃₁ = 0.388 r₃₂ = 58.3720 d₃₂ =4.869 n_(e32) = 1.43985 ν_(e32) = 94.53 r₃₃ = −203.9907 d₃₃ = D33 r₃₄ =∞ d₃₄ = 33.000 n_(e34) = 1.61173 ν_(e34) = 46.30 r₃₅ = ∞ d₃₅ = 13.200n_(e35) = 1.51825 ν_(e35) = 63.93 r₃₆ = ∞ d₃₆ = 0.500 r₃₇ = ∞ (imagingplane) d₃₇ = 0.000 Zoom data 0.3× 0.4× 0.5× D1 43.904 39.311 43.788 D7110.381 79.183 50.950 D15 3.089 38.880 62.637 D22 3.559 5.250 7.195 D3320.639 18.949 17.003 Condition parameters and others Magnification: β0.3× 0.4× 0.5× Entrance pupil position: En 1124.667 16516.516 −1141.823Object-to-image distance: L 429.959 429.959 429.959 |En|/L 2.616 38.4142.656 Exit pupil position: Ex −351.154 −741.700 24496.963 |Ex|/|L/β|0.245 0.690 28.488 F-number: FNO 3.500 3.560 3.646 The amount offluctuation of FNO 0.146 ΔFNO/Δβ 0.729 Radius of curvature on the objectside: R3f 38.452 Radius of curvature on the image side: R3b 17.589|(R3f + R3b)/(R3f − R3b)| 2.686

Third Embodiment

FIGS. 5A, 5B, and 5C show optical arrangements where imagingmagnifications are set to 0.3×, 0.4×, and 0.5×, respectively, in thethird embodiment. FIGS. 6A, 6B, and 6C show aberration characteristicsin focusing of an infinite object point where the imaging magnificationis set to 0.4× in the third embodiment.

The image forming optical system of the third embodiment includes thevariable magnification optical system Z. In this figure, again,reference symbol P represents a prism, CG represents a glass cover, andI represents an imaging plane.

The variable magnification optical system Z comprises, in order form theobject side toward the image side, the first lens unit G1 with positiverefracting power, the second lens unit G2 with positive refractingpower, the third lens unit G3 with negative refracting power, theaperture stop S, and the fourth lens unit G4 with positive refractingpower.

The first lens unit G1 includes the biconvex lens L1 ₁, the negativemeniscus lens L1 ₂′ with a convex surface directed toward the objectside, and a positive meniscus lens L1 ₃′ with a convex surface directedtoward the object side, arranged in this order from the object side.

The second lens unit G2 includes the negative meniscus lens L2 ₁ with aconvex surface directed toward the object side, the biconvex lens L2 ₂,the negative meniscus lens L2 ₃ with a concave surface directed towardthe object side, and the biconvex lens L2 ₄, arranged in this order fromthe object side.

The third lens unit G3 includes the positive meniscus lens L3 ₁ with aconvex surface directed toward the object side, the negative meniscuslens L3 ₂ with a convex surface directed toward the object side, and thenegative meniscus lens L3 ₃ with a convex surface directed toward theobject side, arranged in this order from the object side.

The fourth lens unit G4 includes the cemented lens with the biconcavelens L4 ₁ and the biconvex lens L4 ₂, the biconcave lens L4 ₃, thebiconvex lens L4 ₄, the biconvex lens L4 ₅, and the biconvex lens L4 ₆,arranged in this order from the object side.

When the magnification is changed from 0.3× to 0.5× in focusing of theinfinite object point, the first lens unit G1, after being moved oncetoward the object side, is moved toward the image side; the second lensunit G2 is moved toward the object side; the third lens unit G3 ismoved, together with the stop S, toward the object side so that thespacing between the third lens unit G3 and the fourth lens unit G4 isslightly widened; and the fourth lens unit G4 remains fixed.

The object-to-image distance where the magnification is changed isconstantly maintained.

Subsequently, numerical data of optical members constituting the imageforming optical system of the third embodiment are listed below.

Numerical data 3 Image height: 5.783 r₀ = ∞ (object) d₀ = 50.000 r₁ = ∞(object surface) d₁ = D1 r₂ = 67.5689 d₂ = 7.816 n_(e2) = 1.48915 ν_(e2)= 70.04 r₃ = −335.3716 d₃ = 0.300 r₄ = 140.6380 d₄ = 6.025 n_(e4) =1.61639 ν_(e4) = 44.15 r₅ = 45.2535 d₅ = 8.810 r₆ = 57.6476 d₆ = 11.963n_(e6) = 1.43985 ν_(e6) = 94.53 r₇ = 109.0130 d₇ = D7 r₈ = 140.9050 d₈ =6.209 n_(e8) = 1.61639 ν_(e8) = 44.15 r₉ = 59.1517 d₉ = 5.421 r₁₀ =89.7738 d₁₀ = 7.460 n_(e10) = 1.43985 ν_(e10) = 94.53 r₁₁ = −74.4487 d₁₁= 1.335 r₁₂ = −57.6329 d₁₂ = 7.000 n_(e12) = 1.61639 ν_(e12) = 44.15 r₁₃= −145.4391 d₁₃ = 15.344 r₁₄ = 312.0611 d₁₄ = 8.089 n_(e14) = 1.43985ν_(e14) = 94.53 r₁₅ = −66.7614 d₁₅ = D15 r₁₆ = 42.2336 d₁₆ = 4.331n_(e16) = 1.69417 ν_(e16) = 30.83 r₁₇ = 254.0344 d₁₇ = 0.300 r₁₈ =24.1640 d₁₈ = 9.326 n_(e18) = 1.72538 ν_(e18) = 34.47 r₁₉ = 20.0169 d₁₉= 1.206 r₂₀ = 36.3821 d₂₀ = 2.000 n_(e20) = 1.72538 ν_(e20) = 34.47 r₂₁= 16.7574 d₂₁ = 2.601 r₂₂ = ∞ (aperture stop) d₂₂ = D22 r₂₃ = −26.7471d₂₃ = 2.030 n_(e23) = 1.61669 ν_(e23) = 44.02 r₂₄ = 24.0157 d₂₄ = 5.463n_(e24) = 1.48915 ν_(e24) = 70.04 r₂₅ = −17.6590 d₂₅ = 4.328 r₂₆ =−13.4729 d₂₆ = 1.058 n_(e26) = 161639 ν_(e26) = 44.15 r₂₇ = 93.0104 d₂₇= 4.726 r₂₈ = 913.0291 d₂₈ = 8.540 n_(e28) = 1.43985 ν_(e28) = 94.53 r₂₉= −17.8834 d₂₉ = 0.300 r₃₀ = 81.9603 d₃₀ = 6.985 n_(e30) = 1.43985ν_(e30) = 94.53 r₃₁ = −64.2115 d₃₁ = 3.523 r₃₂ = 60.0466 d₃₂ = 6.110n_(e32) = 1.43985 ν_(e32) = 94.53 r₃₃ = −318.5459 d₃₃ = 19.314 r₃₄ = ∞d₃₄ = 33.000 n_(e34) = 1.61173 ν_(e34) = 46.30 r₃₅ = ∞ d₃₅ = 13.200n_(e35) = 151825 ν_(e35) = 63.93 r₃₆ = ∞ d₃₆ = 0.500 r₃₇ = ∞ (imagingplane) d₃₇ = 0.000 Zoom data 0.3× 0.4× 0.5× D1 50.134 38.319 43.946 D7107.947 77.883 43.657 D15 3.000 42.757 69.242 D22 3.638 5.759 7.874Condition parameters and others Magnification: β 0.3× 0.4× 0.5× Entrancepupil position: En 1271.479 −18393.929 −1095.982 Object-to-imagedistance: L 429.334 429.334 429.334 |En|/L 2.962 42.843 2.553 Exit pupilposition: Ex −362.746 −906.100 4824.866 |Ex|/|L/β| 0.253 0.844 5.619F-number: FNO 3.500 3.593 3.687 The amount of fluctuation of FNO 0.187ΔFNO/Δβ 0.935 Radius of curvature on the object side: R3f 42.234 Radiusof curvature on the image side: R3b 16.757 |(R3f + R3b)/(R3f − R3b)|2.316

Fourth Embodiment

FIGS. 7A, 7B, and 7C show optical arrangements where imagingmagnifications are set to 0.3×, 0.4×, and 0.5×, respectively, in thefourth embodiment. FIGS. 8A, 8B, and 8C show aberration characteristicsin focusing of an infinite object point where the imaging magnificationis set to 0.4× in the fourth embodiment.

The image forming optical system of the fourth embodiment includes thevariable magnification optical system Z. In this figure, again,reference symbol P represents a prism, CG represents a glass cover, andI represents an imaging plane.

The variable magnification optical system Z comprises, in order form theobject side toward the image side, the first lens unit G1 with positiverefracting power, the second lens unit G2 with positive refractingpower, the third lens unit G3 with negative refracting power, theaperture stop S, and the fourth lens unit G4 with positive refractingpower.

The first lens unit G1 includes the biconvex lens L1 ₁the negativemeniscus lens L1 ₂′ with a convex surface directed toward the objectside, and the positive meniscus lens L1 ₃′ with a convex surfacedirected toward the object side, arranged in this order from the objectside.

The second lens unit G2 includes the negative meniscus lens L2 ₁ with aconvex surface directed toward the object side, the biconvex lens L2 ₂,the negative meniscus lens L2 ₃ with a concave surface directed towardthe object side, and the biconvex lens L2 ₄, arranged in this order fromthe object side.

The third lens unit G3 includes the positive meniscus lens L3 ₁ with aconvex surface directed toward the object side, the negative meniscuslens L3 ₂ with a convex surface directed toward the object side, and thenegative meniscus lens L3 ₃ with a convex surface directed toward theobject side, arranged in this order from the object side.

The fourth lens unit G4 includes the cemented lens with the biconcavelens L4 ₁ and the biconvex lens L4 ₂, the biconcave lens L4 ₃, thebiconvex lens L4 ₄, the biconvex lens L4 ₅, and the biconvex lens L4 ₆,arranged in this order from the object side.

When the magnification is changed from 0.3× to 0.5× in focusing of theinfinite object point, the first lens unit G1 is moved toward the imageside; the second lens unit G2 is moved toward the object side; the thirdlens unit G3 is moved toward the image side; and the fourth lens unit G4is moved, together with the stop S, toward the image side so that thespacing between the third lens unit G3 and the fourth lens unit G4 isslightly widened.

The object-to-image distance where the magnification is changed isconstantly maintained.

Subsequently, numerical data of optical members constituting the imageforming optical system of the fourth embodiment are listed below.

Numerical data 4 Image height: 5.783 r₀ = ∞ (object) d₀ = 50.000 r₁ = ∞(object surface) d₁ = D1 r₂ = 107.8560 d₂ = 7.337 n_(e2) = 1.48915ν_(e2) = 70.04 r₃ = −119.7849 d₃ = 3.971 r₄ = 454.1088 d₄ = 7.857 n_(e4)= 1.61639 ν_(e4) = 44.15 r₅ = 49.9355 d₅ = 12.309 r₆ = 64.2291 d₆ =6.018 n_(e6) = 1.43985 ν_(e6) = 94.53 r₇ = 300.8668 d₇ = D7 r₈ =126.3256 d₈ = 6.000 n_(e8) = 1.61639 ν_(e8) = 44.15 r₉ = 56.4062 d₉ =6.775 r₁₀ = 81.4055 d₁₀ = 8.793 n_(e10) = 1.43985 ν_(e10) = 94.53 r₁₁ =−83.1434 d₁₁ = 1.494 r₁₂ = −63.8486 d₁₂ = 7.000 n_(e12) = 1.61639ν_(e12) = 44.15 r₁₃ = −133.7944 d₁₃ = 15.757 r₁₄ = 330.3809 d₁₄ = 7.640n_(e14) = 1.43985 ν_(e14) = 94.53 r₁₅ = −69.3107 d₁₅ = D15 r₁₆ = 40.1299d₁₆ = 4.652 n_(e16) = 1.69417 ν_(e16) = 30.83 r₁₇ = 187.3566 d₁₇ = 0.300r₁₈ = 24.6796 d₁₈ = 9.539 n_(e18) = 1.72538 ν_(e18) = 34.47 r₁₉ =20.3802 d₁₉ = 1.377 r₂₀ = 39.2697 d₂₀ = 2.000 n_(e20) = 1.72538 ν_(e20)= 34.47 r₂₁ = 16.0804 d₂₁ = D21 r₂₂ = ∞ (aperture stop) d₂₂ = 3.575 r₂₃= −30.0984 d₂₃ = 2.000 n_(e23) = 1.61669 ν_(e23) = 44.02 r₂₄ = 23.9795d₂₄ = 8.757 n_(e24) = 1.48915 ν_(e24) = 70.04 r₂₅ = −18.9682 d₂₅ = 3.837r₂₆ = −14.1963 d₂₆ = 0.817 n_(e26) = 1.61639 ν_(e26) = 44.15 r₂₇ =101.4717 d₂₇ = 4.565 r₂₈ = 1012.5847 d₂₈ = 8.419 n_(e28) = 1.43985ν_(e28) = 94.53 r₂₉ = −18.1103 d₂₉ = 0.629 r₃₀ = 69.9749 d₃₀ = 4.880n_(e30) = 1.43985 ν_(e30) = 94.53 r₃₁ = −123.8898 d₃₁ = 0.928 r₃₂ =61.1846 d₃₂ = 4.997 n_(e32) = 1.43985 ν_(e32) = 94.53 r₃₃ = −136.6736d₃₃ = D33 r₃₄ = ∞ d₃₄ = 33.000 n_(e34) = 1.61173 ν_(e34) = 46.30 r₃₅ = ∞d₃₅ = 13.200 n_(e35) = 1.51825 ν_(e35) = 63.93 r₃₆ = ∞ d₃₆ = 0.500 r₃₇ =∞ (imaging surface) d₃₇ = 0.000 Zoom data 0.3× 0.4× 0.5× D1 38.76544.451 53.283 D7 117.344 81.410 52.958 D15 3.000 34.932 56.369 D21 2.6143.787 5.228 D33 21.660 18.803 15.544 Condition parameters and othersMagnification: β 0.3× 0.4× 0.5× Entrance pupil position: En 1117.8285171.585 −1158.986 Object-to-image distance: L 432.125 432.125 432.125|En|/L 2.587 11.968 2.682 Exit pupil position: Ex −357.630 −357.630−357.630 |Ex|/|L/β| 0.248 0.331 0.485 F-number: FNO 3.500 3.479 3.414The amount of fluctuation of FNO −0.046 ΔFNO/Δβ −0.228 Radius ofcurvature on the object side: R3f 40.130 Radius of curvature on theimage side: R3b 16.080 |(R3f + R3b)/(R3f − R3b)| 2.337

Fifth Embodiment

FIGS. 9A, 9B, and 9C show optical arrangements where imagingmagnifications are set to 0.3×, 0.4×, and 0.5×, respectively, in thefifth embodiment. FIGS. 10A, 10B, and 10C show aberrationcharacteristics in focusing of an infinite object point where theimaging magnification is set to 0.4× in the fifth embodiment.

The image forming optical system of the fifth embodiment includes thevariable magnification optical system Z. In this figure, again,reference symbol P represents a prism, CG represents a glass cover, andI represents an imaging plane.

The variable magnification optical system Z comprises, in order form theobject side toward the image side, the first lens unit G1 with positiverefracting power, the second lens unit G2 with positive refractingpower, the third lens unit G3 with negative refracting power, theaperture stop S, and the fourth lens unit G4 with positive refractingpower.

The first lens unit G1 includes a plano-convex lens L1 ₁′ with a convexsurface on the object side and a flat surface on the image side, thenegative meniscus lens L1 ₂′ with a convex surface directed toward theobject side, and the positive meniscus lens L1 ₃′ with a convex surfacedirected toward the object side, arranged in this order from the objectside.

The second lens unit G2 includes the negative meniscus lens L2 ₁ with aconvex surface directed toward the object side, the biconvex lens L2 ₂,the negative meniscus lens L2 ₃ with a concave surface directed towardthe object side, and the biconvex lens L2 ₄, arranged in this order fromthe object side.

The third lens unit G3 includes the positive meniscus lens L3 ₁ with aconvex surface directed toward the object side, the negative meniscuslens L3 ₂ with a convex surface directed toward the object side, and thenegative meniscus lens L3 ₃ with a convex surface directed toward theobject side, arranged in this order from the object side.

The fourth lens unit G4 includes the cemented lens with the biconcavelens L4 ₁ and the biconvex lens L4 ₂, the biconcave lens L4 ₃, thebiconvex lens L4 ₄, the biconvex lens L4 ₅, and the biconvex lens L4 ₆,arranged in this order from the object side.

When the magnification is changed from 0.3× to 0.5× in focusing of theinfinite object point, the first lens unit G1, after being moved oncetoward the object side, is moved toward the image side; the second lensunit G2 is moved toward the object side; the third lens unit G3 is movedtoward the object side so that the spacing between the third lens unitG3 and the fourth lens unit G4 is slightly widened; and the fourth lensunit G4 remains fixed, together with the stop S.

The object-to-image distance where the magnification is changed isconstantly maintained.

Subsequently, numerical data of optical members constituting the imageforming optical system of the fifth embodiment are listed below.

Numerical data 5 Image height: 5.783 r₀ = ∞ (object) d₀ = 50.000 r₁ = ∞(object surface) d₁ = D1 r₂ = 53.6678 d₂ = 7.850 n_(e2) = 1.48915 ν_(e2)= 70.04 r₃ = ∞ d₃ = 0.300 r₄ = 74.4381 d₄ = 6.000 n_(e4) = 1.61639ν_(e4) = 44.15 r₅ = 34.5362 d₅ = 8.043 r₆ = 39.1043 d₆ = 4.857 n_(e6) =1.43985 ν_(e6) = 94.53 r₇ = 52.1576 d₇ = D7 r₈ = 149.0540 d₈ = 6.000n_(e8) = 1.61639 ν_(e8) = 44.15 r₉ = 50.6084 d₉ = 6.908 r₁₀ = 78.4447d₁₀ = 9.096 n_(e10) = 1.43985 ν_(e10) = 94.53 r₁₁ = −67.1214 d₁₁ = 1.239r₁₂ = −55.5198 d₁₂ = 7.000 n_(e12) = 1.61639 ν_(e12) = 44.15 r₁₃ =−130.4767 d₁₃ = 17.549 r₁₄ = 526.4312 d₁₄ = 10.495 n_(e14) = 1.43985ν_(e14) = 94.53 r₁₅ = −60.7655 d₁₅ = D15 r₁₆ = 42.8799 d₁₆ = 4.607n_(e16) = 1.69417 ν_(e16) = 30.83 r₁₇ = 241.5957 d₁₇ = 0.300 r₁₈ =24.0062 d₁₈ = 9.266 n_(e18) = 1.72538 ν_(e18) = 34.47 r₁₉ = 20.0630 d₁₉= 1.423 r₂₀ = 37.0493 d₂₀ = 2.000 n_(e20) = 1.72538 ν_(e20) = 34.47 r₂₁= 16.8163 d₂₁ = D21 r₂₂ = ∞ (aperture stop) d₂₂ = 3.685 r₂₃ = −27.7248d₂₃ = 2.000 n_(e23) = 1.61669 ν_(e23) = 44.02 r₂₄ = 25.1231 d₂₄ = 5.991n_(e24) = 1.48915 ν_(e24) = 70.04 r₂₅ = −18.8837 d₂₅ = 4.943 r₂₆ =−14.1386 d₂₆ = 0.553 n_(e26) = 1.61639 ν_(e26) = 44.15 r₂₇ = 103.4372d₂₇ = 4.610 r₂₈ = 946.2142 d₂₈ = 8.426 n_(e28) = 1.43985 ν_(e28) = 94.53r₂₉ = −18.1453 d₂₉ = 0.300 r₃₀ = 79.1515 d₃₀ = 7.210 n_(e30) = 1.43985ν_(e30) = 94.53 r₃₁ = −65.2376 d₃₁ = 5.640 r₃₂ = 63.0290 d₃₂ = 6.581n_(e32) = 1.43985 ν_(e32) = 94.53 r₃₃ = −291.4522 d₃₃ = 19.405 r₃₄ = ∞d₃₄ = 33.000 n_(e34) = 1.61173 ν_(e34) = 46.30 r₃₅ = ∞ d₃₅ = 13.200n_(e35) = 1.51825 ν_(e35) = 63.93 r₃₆ = ∞ d₃₆ = 0.500 r₃₇ = ∞ (imagingsurface) d₃₇ = 0.000 Zoom data 0.3× 0.4× 0.5× D1 42.960 38.372 47.817 D7105.480 70.527 33.769 D15 3.000 40.551 66.211 D21 2.679 4.670 6.322Condition parameters and others Magnification: β 0.3× 0.4× 0.5× Entrancepupil position: En 1295.110 24846.034 −1103.070 object-to-imagedistance: L 423.096 423.096 423.096 |En|/L 3.061 58.724 2.607 Exit pupilposition: Ex −366.274 −366.274 −366.274 |Ex|/|L/β| 0.260 0.346 0.433F-number: FNO 3.500 3.500 3.500 The amount of fluctuation of FNO 0.000ΔFNO/Δβ −0.002 Radius of curvature on the object side: R3f 42.880 Radiusof curvature on the image side: R3b 16.816 |(R3f + R3b)/(R3f − R3b)|2.290

Sixth Embodiment

FIGS. 11A, 11B, and 11C show optical arrangements where imagingmagnifications are set to 0.3×, 0.4×, and 0.5×, respectively, in thesixth embodiment. FIGS. 12A, 12B, and 12C show aberrationcharacteristics in focusing of an infinite object point where theimaging magnification is set to 0.4× in the sixth embodiment.

The image forming optical system of the sixth embodiment includes thevariable magnification optical system Z. In this figure, again,reference symbol P represents a prism, CG represents a glass cover, andI represents an imaging plane.

The variable magnification optical system Z comprises, in order form theobject side toward the image side, the first lens unit G1 with positiverefracting power, the second lens unit G2 with positive refractingpower, the third lens unit G3 with negative refracting power, theaperture stop S, and the fourth lens unit G4 with positive refractingpower.

The first lens unit G1 includes the biconvex lens L1 ₁, the biconcavelens L1 ₂, and the biconvex lens L1 ₃, arranged in this order from theobject side.

The second lens unit G2 includes the negative meniscus lens L2 ₁ with aconvex surface directed toward the object side, the biconvex lens L2 ₂,the negative meniscus lens L2 ₃ with a concave surface directed towardthe object side, and a positive meniscus lens L2 ₄′ with a concave lensdirected toward the object side, arranged in this order from the objectside.

The third lens unit G3 includes the positive meniscus lens L3 ₁ with aconvex surface directed toward the object side, the negative meniscuslens L3 ₂ with a convex surface directed toward the object side, and thenegative meniscus lens L3 ₃ with a convex surface directed toward theobject side, arranged in this order from the object side.

The fourth lens unit G4 includes the cemented lens with the biconcavelens L4 ₁ and the biconvex lens L4 ₂, the biconcave lens L4 ₃, thebiconvex lens L4 ₄, the biconvex lens L4 ₅, and the biconvex lens L4 ₆,arranged in this order from the object side.

When the magnification is changed from 0.3× to 0.5× in focusing of theinfinite object point, the first lens unit G1 is moved toward the objectside; the second lens unit G2 is moved toward the object side so thatspacing between the first lens unit G1 and the second lens unit G2 iswidened; the third lens unit G3 is moved, together with the stop S,toward the image side; and the fourth lens unit G4 is moved toward theimage side so that the spacing between the third lens unit G3 and thefourth lens unit G4 is slightly widened.

The object-to-image distance where the magnification is changed isconstantly maintained.

Subsequently, numerical data of optical members constituting the imageforming optical system of the sixth embodiment are listed below.

Numerical data 6 Image height: 5.783 r₀ = ∞ (object) d₀ = 50.000 r₁ = ∞(object surface) d₁ = D1 r₂ = 361.3250 d₂ = 12.000 n_(e2) = 1.48915ν_(e2) = 70.04 r₃ = −65.3190 d₃ = 0.300 r₄ = −90.3503 d₄ = 8.000 n_(e4)= 1.61639 ν_(e4) = 44.15 r₅ = 45.5593 d₅ = 11.355 r₆ = 65.7955 d₆ =12.000 n_(e6) = 1.43985 ν_(e6) = 94.53 r₇ = −101.4028 d₇ = D7 r₈ =113.0032 d₈ = 7.000 n_(e8) = 1.61639 ν_(e8) = 44.15 r₉ = 53.1618 d₉ =7.854 r₁₀ = 84.6315 d₁₀ = 8.348 n_(e10) = 1.43985 ν_(e10) = 94.53 r₁₁ =−82.9242 d₁₁ = 2.346 r₁₂ = −51.6817 d₁₂ = 6.901 n_(e12) = 1.61639ν_(e12) = 44.15 r₁₃ = −78.9538 d₁₃ = 0.300 r₁₄ = −746.1406 d₁₄ = 7.363n_(e14) = 1.43985 ν_(e14) = 94.53 r₁₅ = −54.9986 d₁₅ = D15 r₁₆ = 40.2152d₁₆ = 4.672 n_(e16) = 1.69417 ν_(e16) = 30.83 r₁₇ = 202.9669 d₁₇ = 0.300r₁₈ = 25.2156 d₁₈ = 9.337 n_(e18) = 1.72538 ν_(e18) = 34.47 r₁₉ =20.5989 d₁₉ = 1.486 r₂₀ = 47.2290 d₂₀ = 2.000 n_(e20) = 1.72538 ν_(e20)= 34.47 r₂₁ = 17.1952 d₂₁ = D21 r₂₂ = ∞ (aperture stop) d₂₂ = 8.090 r₂₃= −31.8155 d₂₃ = 12.000 n_(e23) = 1.61669 ν_(e23) = 44.02 r₂₄ = 23.4115d₂₄ = 6.316 n_(e24) = 1.48915 ν_(e24) = 70.04 r₂₅ = −23.1015 d₂₅ = 1.525r₂₆ = −17.3296 d₂₆ = 0.137 n_(e26) = 1.61639 ν_(e26) = 44.15 r₂₇ =121.5936 d₂₇ = 4.365 r₂₈ = 236.9154 d₂₈ = 8.477 n_(e28) = 1.43985ν_(e28) = 94.53 r₂₉ = −20.8758 d₂₉ = 0.300 r₃₀ = 78.3373 d₃₀ = 5.274n_(e30) = 1.43985 ν_(e30) = 94.53 r₃₁ = −103.6059 d₃₁ = 0.983 r₃₂ =81.5041 d₃₂ = 5.879 n_(e32) = 1.43985 ν_(e32) = 94.53 r₃₃ = −103.9512d₃₃ = D33 r₃₄ = ∞ d₃₄ = 33.000 n_(e34) = 1.61173 ν_(e34) = 46.30 r₃₅ = ∞d₃₅ = 13.200 n_(e35) = 1.51825 ν_(e35) = 63.93 r₃₆ = ∞ d₃₆ = 0.500 r₃₇ =∞ (imaging surface) d₃₇ = 0.000 Zoom data 0.3× 0.4× 0.5× D1 68.66851.352 36.703 D7 65.281 56.350 50.311 D15 3.000 32.024 53.396 D21 2.7702.825 3.398 D33 20.686 17.854 16.597 Condition parameters and othersMagnification: β 0.3× 0.4× 0.5× Entrance pupil position: En 140.733198.229 329.610 Object-to-image distance: L 412.012 412.012 412.012|En|/L 0.342 0.481 0.800 Exit pupil position: Ex 2022.944 2022.9442022.944 |Ex|/|L/β| 1.473 1.964 2.455 F-number: FNO 3.500 3.511 3.516The amount of fluctuation of FNO 0.016 ΔFNO/Δβ 0.082 Radius of curvatureon the object side: R3f 40.215 Radius of curvature on the image side:R3b 17.195 |(R3f + R3b)/(R3f − R3b)| 2.494

Seventh Embodiment

FIGS. 13A, 13B, and 13C show optical arrangements where imagingmagnifications are set to 0.3×, 0.4×, and 0.5×, respectively, in theseventh embodiment. FIGS. 14A, 14B, and 14C show aberrationcharacteristics in focusing of an infinite object point where theimaging magnification is set to 0.4× in the seventh embodiment.

The image forming optical system of the seventh embodiment includes thevariable magnification optical system Z. In this figure, referencesymbol GL represents a plane-parallel plate, P1 and P2 represent prisms,CG represents a glass cover, and I represents an imaging plane.

The variable magnification optical system Z comprises, in order form theobject side toward the image side, the first lens unit G1 with positiverefracting power, the second lens unit G2 with positive refractingpower, the third lens unit G3 with negative refracting power, theaperture stop S, and the fourth lens unit G4 with positive refractingpower.

The first lens unit G1 includes the biconvex lens L1 ₁, the negativemeniscus lens L1 ₂′ with a convex surface directed toward the objectside, and the positive meniscus lens L1 ₃′ with a convex surfacedirected toward the object side, arranged in this order from the objectside.

The second lens unit G2 includes the negative meniscus lens L2 ₁ with aconvex surface directed toward the object side, the biconvex lens L2 ₂,the negative meniscus lens L2 ₃ with a concave surface directed towardthe object side, and the biconvex lens L2 ₄, arranged in this order fromthe object side.

The third lens unit G3 includes the positive meniscus lens L3 ₁ with aconvex surface directed toward the object side, the negative meniscuslens L3 ₂ with a convex surface directed toward the object side, and thenegative meniscus lens L3 ₃ with a convex surface directed toward theobject side, arranged in this order from the object side.

The fourth lens unit G4 includes the cemented lens with the biconcavelens L4 ₁ and the biconvex lens L4 ₂, the biconcave lens L4 ₃, apositive meniscus lens L4 ₄′ with a concave surface directed toward theobject side, the biconvex lens L4 ₅, and the biconvex lens L4 ₆,arranged in this order from the object side.

When the magnification is changed from 0.3× to 0.5× in focusing of theinfinite object point, the first lens unit G1 is moved toward the objectside; the second lens unit G2 is moved toward the object side so thatthe spacing between the first lens unit G1 and the second lens unit G2is narrowed; the third lens unit G3 is moved, together with the stop S,toward the image side; and the fourth lens unit G4 is moved toward theimage side so that the spacing between the third lens unit G3 and thefourth lens unit G4 is slightly widened.

The object-to-image distance where the magnification is changed isconstantly maintained.

Subsequently, numerical data of optical members constituting the imageforming optical system of the seventh embodiment are listed below.

Numerical data 7 Image height: 5.783 r₀ = ∞ (object) d₀ = 51.000 r₁ = ∞(object surface) d₁ = 9.260 n_(e1) = 1.51825 ν_(e1) = 63.93 r₂ = ∞ d₂ =2.740 r₃ = ∞ d₃ = 35.000 r₄ = ∞ d₄ = 60.000 n_(e4) = 1.51825 ν_(e4) =63.93 r₅ = ∞ d₅ = D5 r₆ = 206.3131 d₆ = 6.508 n_(e6) = 1.48915 ν_(e6) =70.04 r₇ = −156.0897 d₇ = 15.114 r₈ = 130.1657 d₈ = 8.000 n_(e8) =1.61639 ν_(e8) = 44.15 r₉ = 61.3830 d₉ = 1.693 r₁₀ = 80.8720 d₁₀ =12.000 n_(e10) = 1.43985 ν_(e10) = 94.53 r₁₁ = 232.8980 d₁₁ = D11 r₁₂ =672.7620 d₁₂ = 6.836 n_(e12) = 1.61639 ν_(e12) = 44.15 r₁₃ = 82.8549 d₁₃= 2.818 r₁₄ = 110.5678 d₁₄ = 9.282 n_(e14) = 1.43985 ν_(e14) = 94.53 r₁₅= −65.4332 d₁₅ = 0.300 r₁₆ = −67.0268 d₁₆ = 6.107 n_(e16) = 1.61639ν_(e16) = 44.15 r₁₇ = −156.9702 d₁₇ = 50.171 r₁₈ = 160.2358 d₁₈ = 10.874n_(e18) = 1.43985 ν_(e18) = 94.53 r₁₉ = −98.7058 d₁₉ = D19 r₂₀ = 37.4259d₂₀ = 5.034 n_(e20) = 1.69417 ν_(e20) = 30.83 r₂₁ = 212.9113 d₂₁ = 0.300r₂₂ = 22.9775 d₂₂ = 8.363 n_(e22) = 1.72538 ν_(e22) = 34.47 r₂₃ =18.2286 d₂₃ = 1.827 r₂₄ = 101.2051 d₂₄ = 2.247 n_(e24) = 1.72538 ν_(e24)= 34.47 r₂₅ = 17.6992 d₂₅ = 2.554 r₂₆ = ∞ () d₂₆ = D26 r₂₇ = −55.3149d₂₇ = 2.589 n_(e27) = 1.61669 ν_(e27) = 44.02 r₂₈ = 20.3875 d₂₈ = 11.136n_(e28) = 1.48915 ν_(e28) = 70.04 r₂₉ = −22.7793 d₂₉ = 2.967 r₃₀ =−17.4070 d₃₀ = 2.255 n_(e30) = 1.61639 ν_(e30) = 44.15 r₃₁ = 660.0000d₃₁ = 5.164 r₃₂ = −361.4116 d₃₂ = 9.280 n_(e32) = 1.43985 ν_(e32) =94.53 r₃₃ = −21.6618 d₃₃ = 0.300 r₃₄ = 57.4166 d₃₄ = 5.104 n_(e34) =1.43985 ν_(e34) = 94.53 r₃₅ = −177.5066 d₃₅ = 0.350 r₃₆ = 61.7155 d₃₆ =4.849 n_(e36) = 1.43985 ν_(e36) = 94.53 r₃₇ = −672.7620 d₃₇ = D37 r₃₈ =∞ d₃₈ = 33.000 n_(e38) = 1.61173 ν_(e38) = 46.30 r₃₉ = ∞ d₃₉ = 13.200n_(e39) = 1.51825 ν_(e39) = 63.93 r₄₀ = ∞ d₄₀ = 0.500 r₄₁ = ∞ (imagingplane) d₄₁ = 0.000 Zoom data 0.3× 0.4× 0.5× D5 32.142 28.009 24.962 D1158.194 27.683 8.473 D19 3.000 39.963 64.634 D26 3.340 5.440 7.048 D3723.777 19.357 15.336 Condition parameters and others Magnification: β0.3× 0.4× 0.5× Entrance pupil position: En 1215.330 17052.978 −1195.682Object-to-image distance: L 467.675 467.675 467.675 |En|/L 2.599 36.4632.557 Exit pupil position: Ex −361.027 −890.944 −13016.681 |Ex|/|L/β|0.232 0.762 13.916 F-number: FNO 3.500 3.517 3.556 The amount offluctuation of FNO 0.056 ΔFNO/Δβ 0.280 Radius of curvature on the objectside: R3f 37.426 Radius of curvature on the image side: R3b 17.699|(R3f + R3b)/(R3f − R3b)| 2.794

Eighth Embodiment

FIGS. 15A, 15B, and 15C show optical arrangements where imagingmagnifications are set to 0.3×, 0.4×, and 0.5×, respectively, in theeighth embodiment. FIGS. 16A, 16B, and 16C show aberrationcharacteristics in focusing of an infinite object point where theimaging magnification is set to 0.4× in the eighth embodiment.

The image forming optical system of the eighth embodiment includes thevariable magnification optical system Z. In this figure, again,reference symbol GL represents a plane-parallel plate, P1 and P2represent prisms, CG represents a glass cover, and I represents animaging plane.

The variable magnification optical system Z comprises, in order form theobject side toward the image side, the first lens unit G1 with positiverefracting power, the second lens unit G2 with positive refractingpower, the third lens unit G3 with negative refracting power, theaperture stop S, and the fourth lens unit G4 with positive refractingpower.

The first lens unit G1 includes a positive meniscus lens L1 ₁″ with aconcave surface directed toward the object side, the negative meniscuslens L1 ₂′ with a concave surface directed toward the object side, andthe biconvex lens L1 ₃, arranged in this order from the object side.

The second lens unit G2 includes the negative meniscus lens L2 ₁ with aconvex surface directed toward the object side, the biconvex lens L2 ₂,the negative meniscus lens L2 ₃ with a concave surface directed towardthe object side, a positive meniscus lens L2 ₄′ with a concave surfacedirected toward the object side, and a positive meniscus lens L2 ₅ witha convex surface directed toward the object side, arranged in this orderfrom the object side.

The third lens unit G3 includes a biconvex lens L3 ₁′, the negativemeniscus lens L3 ₂ with a convex surface directed toward the objectside, and the negative meniscus lens L3 ₃ with a convex surface directedtoward the object side, arranged in this order from the object side.

The fourth lens unit G4 includes a negative meniscus lens L4 ₁′ with aconvex surface directed toward the object side, a positive meniscus lensL4 ₂′ with a concave surface directed toward the object side, a negativemeniscus lens L4 ₃′ with a concave surface directed toward the objectside, the positive meniscus lens L4 ₄′ with a concave surface directedtoward the object side, the biconvex lens L4 ₅, and a positive meniscuslens L4 ₆′ with a convex surface directed toward the object side,arranged in this order from the object side.

When the magnification is changed from 0.3× to 0.5× in focusing of theinfinite object point, the first lens unit G1 is moved toward the objectside; the second lens unit G2 is moved toward the object side so thatthe spacing between the first lens unit G1 and the second lens unit G2is widened; the third lens unit G3 is moved, together with the stop S,toward the object side so that spacing between the second lens unit G2and the third lens unit G3 is slightly widened; and the fourth lens unitG4, after being slightly moved once toward the image side, is slightlymoved toward the object side.

The object-to-image distance where the magnification is changed isconstantly maintained.

Subsequently, numerical data of optical members constituting the imageforming optical system of the eighth embodiment are listed below.

Numerical data 8 Image height: 5.783 r₀ = ∞ (object) d₀ = 51.000 r₁ = ∞(object surface) d₁ = 9.260 n_(e1) = 1.51825 ν_(e1) = 63.93 r₂ = ∞ d₂ =2.740 r₃ = ∞ d₃ = 35.000 r₄ = ∞ d₄ = 60.000 n_(e4) = 1.51825 ν_(e4) =63.93 r₅ = ∞ d₅ = D5 r₆ = −218.393 d₆ = 11.966 n_(e6) = 1.48915 ν_(e6) =70.04 r₇ = −59.981 d₇ = 0.724 r₈ = −58.074 d₈ = 8.000 n_(e8) = 1.61639ν_(e8) = 44.15 r₉ = −192.015 d₉ = 0.300 r₁₀ = 453.258 d₁₀ = 11.399n_(e10) = 1.43985 ν_(e10) = 94.53 r₁₁ = −95.008 d₁₁ = D11 r₁₂ = 111.240d₁₂ = 6.982 n_(e12) = 1.61639 ν_(e12) = 44.15 r₁₃ = 49.021 d₁₃ = 0.808r₁₄ = 52.125 d₁₄ = 6.307 n_(e14) = 1.43985 ν_(e14) = 94.53 r₁₅ =−602.409 d₁₅ = 3.345 r₁₆ = −51.702 d₁₆ = 7.000 n_(e16) = 1.61639 ν_(e16)= 44.15 r₁₇ = −123.131 d₁₇ = 0.300 r₁₈ = −267.367 d₁₈ = 5.244 n_(e18) =1.43985 ν_(e18) = 94.53 r₁₉ = −59.230 d₁₉ = 0.300 r₂₀ = 62.890 d₂₀ =5.562 n_(e20) = 1.43985 ν_(e20) = 94.53 r₂₁ = 208.855 d₂₁ = D21 r₂₂ =109.670 d₂₂ = 4.560 ν_(e22) = 1.67765 ν_(e22) = 31.84 r₂₃ = −261.555 d₂₃= 4.236 r₂₄ = 27.656 d₂₄ = 9.660 n_(e24) = 1.83945 ν_(e24) = 42.47 r₂₅ =22.416 d₂₅ = 3.719 r₂₆ = 591.785 d₂₆ = 2.000 n_(e26) = 1.83945 ν_(e26) =42.47 r₂₇ = 32.027 d₂₇ = 2.504 r₂₈ = ∞ (aperture stop) d₂₈ = D28 r₂₉ =235.972 d₂₉ = 3.058 n_(e29) = 1.61639 ν_(e29) = 44.15 r₃₀ = 39.062 d₃₀ =3.236 r₃₁ = −23.495 d₃₁ = 6.117 n_(e31) = 1.43985 ν_(e31) = 94.53 r₃₂ =−17.821 d₃₂ = 0.300 r₃₃ = −18.080 d₃₃ = 4.802 n_(e33) = 1.61639 ν_(e33)= 44.15 r₃₄ = −31.126 d₃₄ = 0.300 r₃₅ = −67.557 d₃₅ = 4.329 n_(e35) =1.43985 ν_(e35) = 94.53 r₃₆ = −32.513 d₃₆ = 0.300 r₃₇ = 81.623 d₃₇ =4.159 n_(e37) = 1.43985 ν_(e37) = 94.53 r₃₈ = −357.038 d₃₈ = 0.484 r₃₉ =34.763 d₃₉ = 5.000 n_(e39) = 1.43985 ν_(e39) = 94.53 r₄₀ = 244.020 d₄₀ =D40 r₄₁ = ∞ d₄₁ = 33.000 n_(e41) = 1.61173 ν_(e41) = 46.30 r₄₂ = ∞ d₄₂ =13.200 n_(e42) = 1.51825 ν_(e42) = 63.93 r₄₃ = ∞ d₄₃ = 0.500 r₄₄ = ∞(imaging plane) d₄₄ = 0 Zoom data 0.3× 0.4× 0.5× D5 193.324 142.89590.403 D11 3.000 43.660 80.930 D21 3.160 6.077 8.978 D28 20.516 27.62834.649 D40 11.289 11.032 16.330 Condition parameters and othersMagnification: β 0.3× 0.4× 0.5× Entrance pupil position: En 89.768209.179 450.391 Object-to-image distance: L 562.991 562.991 562.991|En|/L 0.159 0.372 0.800 Exit pupil position: Ex −355.985 −5834.634634.502 |Ex|/|L/β| 0.190 4.145 0.564 F-number: FNO 3.500 3.789 4.037 Theamount of fluctuation of FNO 0.537 ΔFNO/Δβ 2.685 Radius of curvature onthe object side: R3f 109.670 Radius of curvature on the image side: R3b32.027 |(R3f + R3b)/(R3f − R3b)| 1.825

Ninth Embodiment

FIGS. 17A, 17B, and 17C show optical arrangements where imagingmagnifications are set to 0.3×, 0.4×, and 0.5×, respectively, in theninth embodiment. FIGS. 18A, 18B, and 18C show aberrationcharacteristics in focusing of an infinite object point where theimaging magnification is set to 0.4× in the ninth embodiment.

The image forming optical system of the ninth embodiment includes thevariable magnification optical system Z. In this figure, again,reference symbol GL represents a plane-parallel plate, P1 and P2represent prisms, CG represents a glass cover, and I represents animaging plane.

The variable magnification optical system Z comprises, in order form theobject side toward the image side, the first lens unit G1 with positiverefracting power, the second lens unit G2 with positive refractingpower, the third lens unit G3 with negative refracting power, theaperture stop S, and the fourth lens unit G4 with positive refractingpower.

The first lens unit G1 includes the positive meniscus lens L1 ₁″ with aconcave surface directed toward the object side, the negative meniscuslens L1 ₂′ with a concave surface directed toward the object side, and apositive meniscus lens L1 ₃″ with a concave surface directed toward theobject side, arranged in this order from the object side.

The second lens unit G2 includes the negative meniscus lens L2 ₁ with aconvex surface directed toward the object side, the biconvex lens L2 ₂,the negative meniscus lens L2 ₃ with a concave surface directed towardthe object side, the biconvex lens L2 ₄, and a biconvex lens L2 ₅′,arranged in this order from the object side.

The third lens unit G3 includes the biconvex lens L3 ₁, the negativemeniscus lens L3 ₂ with a convex surface directed toward the objectside, and the negative meniscus lens L3 ₃ with a convex surface directedtoward the object side, arranged in this order from the object side.

The fourth lens unit G4 includes a negative meniscus lens L4 ₁″ with aconcave surface directed toward the object side, the positive meniscuslens L4 ₂′ with a concave surface directed toward the object side, thenegative meniscus lens L4 ₃′ with a concave surface directed toward theobject side, the positive meniscus lens L4 ₄′ with a concave surfacedirected toward the object side, the biconvex lens L4 ₅, and thebiconvex lens L4 ₆, arranged in this order from the object side.

When the magnification is changed from 0.3× to 0.5× in focusing of theinfinite object point, the first lens unit G1 is moved toward the objectside; the second lens unit G2 is moved toward the object side so thatthe spacing between the first lens unit G1 and the second lens unit G2is widened; the third lens unit G3 is moved, together with the stop S,toward the object side so that the spacing between the second lens unitG2 and the third lens unit G3 is widened; and the fourth lens unit G4,after being slightly moved once toward the image side, is slightly movedtoward the object side.

The object-to-image distance where the magnification is changed isconstantly maintained.

Subsequently, numerical data of optical members constituting the imageforming optical system of the ninth embodiment are listed below.

Numerical data 9 Image height: 5.783 r₀ = ∞ (object) d₀ = 21.000 r₁ = ∞(object) d₁ = 26.161 r₂ = ∞ (object surface) d₂ = D2 r₃ = −153.3010 d₃ =12.000 n_(e3) = 1.48915 ν_(e3) = 70.04 r₄ = −56.0044 d₄ = 6.782 r₅ =−42.5771 d₅ = 8.000 n_(e6) = 1.61639 ν_(e5) = 44.15 r₆ = −173.4981 d₆ =15.255 r₇ = −454.5776 d₇ = 12.000 n_(e7) = 1.43985 ν_(e7) = 94.53 r₈ =−54.2450 d₈ = D8 r₉ = 74.1238 d₉ = 7.000 n_(e9) = 1.61639 ν_(e9) = 44.15r₁₀ = 47.9620 d₁₀ = 0.782 r₁₁ = 50.6461 d₁₁ = 6.639 n_(e11) = 1.43985ν_(e11) = 94.53 r₁₂ = −395.4325 d₁₂ = 2.526 r₁₃ = −67.4730 d₁₃ = 6.000n_(e13) = 1.61639 ν_(e13) = 44.15 r₁₄ = −489.0704 d₁₄ = 0.300 r₁₅ =162.7339 d₁₅ = 5.252 n_(e15) = 1.43985 ν_(e15) = 94.53 r₁₆ = −122.6735d₁₆ = 0.300 r₁₇ = 377.7299 d₁₇ = 4.142 n_(e17) = 1.43985 ν_(e17) = 94.53r₁₈ = −202.1041 d₁₈ = D18 r₁₉ = 108.3047 d₁₉ = 4.106 n_(e19) = 1.67765ν_(e19) = 31.84 r₂₀ = −192.0405 d₂₀ = 0.454 r₂₁ = 25.9085 d₂₁ = 9.623n_(e21) = 1.83945 ν_(e21) = 42.47 r₂₂ = 24.8614 d₂₂ = 2.939 r₂₃ =50.8391 d₂₃ = 2.000 n_(e23) = 1.83945 ν_(e23) = 42.47 r₂₄ = 18.5107 d₂₄= 3.223 r₂₅ = ∞ (aperture stop) d₂₅ = D25 r₂₆ = −23.8975 d₂₆ = 8.198n_(e26) = 1.61639 ν_(e26) = 44.15 r₂₇ = −142.2318 d₂₇ = 1.569 r₂₈ =−27.6769 d₂₈ = 12.000 n_(e28) = 1.43985 ν_(e28) = 94.53 r₂₉ = −15.4629d₂₉ = 0.617 r₃₀ = −15.4255 d₃₀ = 2.000 n_(e30) = 1.61639 ν_(e30) = 44.15r₃₁ = −31.9175 d₃₁ = 0.300 r₃₂ = −193.4359 d₃₂ = 5.561 n_(e32) = 1.43985ν_(e32) = 94.53 r₃₃ = −30.6965 d₃₃ = 0.300 r₃₄ = 190.3831 d₃₄ = 4.818n_(e34) = 1.43985 ν_(e34) = 94.53 r₃₅ = −61.6979 d₃₅ = 0.300 r₃₆ =63.1906 d₃₆ = 4.652 n_(e36) = 1.43985 ν_(e36) = 94.53 r₃₇ = −264.7349d₃₇ = D37 r₃₈ = ∞ d₃₈ = 33.000 n_(e38) = 1.61173 ν_(e38) = 46.30 r₃₉ = ∞d₃₉ = 13.200 n_(e39) = 1.51825 ν_(e39) = 63.93 r₄₀ = ∞ d₄₀ = 0.500 r₄₁ =∞ (imaging plane) d₄₁ = 0.000 Zoom data 0.3× 0.4× 0.5× D2 131.948109.433 66.283 D8 3.000 7.576 32.565 D18 3.338 20.375 31.678 D25 6.47011.057 13.774 D37 16.892 13.207 17.349 Condition parameters and othersMagnification: β 0.3× 0.4× 0.5× Entrance pupil position: En 104.859165.265 302.380 Object-to-image distance: L 405.147 405.147 405.147|En|/L 0.259 0.408 0.746 Exit pupil position: Ex −368.020 2564.601598.424 |Ex|/|L/β| 0.273 2.532 0.739 F-number: FNO 3.500 3.725 3.839 Theamount of fluctuation of FNO 0.339 ΔFNO/Δβ 1.693 Radius of curvature onthe object side: R3f 108.305 Radius of curvature on the image side: R3b18.511 |(R3f + R3b)/(R3f − R3b)| 1.412

Subsequently, in the above embodiments, the parameter values of theconditions and whether the lens arrangements satisfy the requirements ofthe present invention are shown in Tables 1-3.

TABLE 1 1st embodiment 2nd embodiment 3rd embodiment Object-sidetelecentricity |En|/L (β = 0.3) 2.71 2.62 2.96 Object-sidetelecentricity |En|/L (β = 0.4) 47.27 38.41 42.84 Object-sidetelecentricity |En|/L (β = 0.5) 2.65 2.66 2.55 Image-side telecentricity|En|/|L/β| (β = 0.3) 0.25 0.25 0.25 Image-side telecentricity |En|/|L/β|(β = 0.4) 0.54 0.69 0.84 Image-side telecentricity |En|/|L/β| (β = 0.5)2.12 28.49 5.62 Conditions (1) and (2) ◯ ◯ ◯ Conditions (1′) and (2′) ◯◯ ◯ Conditions (1″) and (2″) ◯ ◯ ◯ Difference in object-to-imagedistance between 0.3× and 0.5× 0.00000 0.00002 0.00000 Smallestobject-side F-number, MAXFNO 3.5 3.5 3.5 |ΔFNO/Δβ| 0.49 0.729 0.935Conditions (3) and (4) ◯ ◯ ◯ Conditions (3′) and (4′) ◯ ◯ ◯ Conditions(3″) and (4″) ◯ ◯ ◯ Lens arrangement of 1st lens unit: positive ◯ ◯ ◯Lens arrangement of 1st lens unit: positive, negative ◯ ◯ ◯ Lensarrangement of 1st lens unit: positive, negative, positive ◯ ◯ ◯ 3rdlens unit virtual shape factor 2.27 2.69 2.32 |(R3f + R3b)|/|(R3f −R3b)| Condition (5) ◯ ◯ ◯ Condition (5′) ◯ ◯ ◯ Condition (5″) ◯ ◯ ◯ 3rdlens unit: at least two meniscus lenses, each with a convex ◯ ◯ ◯surface directed toward the object side 3rd lens unit: at least threemeniscus lenses, each with a convex ◯ ◯ ◯ surface directed toward theobject side Note: ◯ indicates that conditions are satisfied.

TABLE 2 4th embodiment 5th embodiment 6th embodiment Object-sidetelecentricity |En|/L (β = 0.3) 2.59 3.06 0.34 Object-sidetelecentricity |En|/L (β = 0.4) 11.97 58.72 0.48 Object-sidetelecentricity |En|/L (β = 0.5) 2.68 2.61 0.80 Image-side telecentricity|En|/|L/β| (β = 0.3) 0.25 0.26 1.47 Image-side telecentricity |En|/|L/β|(β = 0.4) 0.33 0.35 1.96 Image-side telecentricity |En|/|L/β| (β = 0.5)0.41 0.43 2.46 Conditions (1) and (2) ◯ ◯ ◯ Conditions (1′) and (2′) X X◯ Conditions (1″) and (2″) X X X Difference in object-to-image distancebetween 0.3× and 0.5× 0.00000 0.00000 0.00000 Smallest object-sideF-number, MAXFNO 3.45 3.5 3.5 |ΔFNO/Δβ| 0.228 0.002 0.082 Conditions (3)and (4) ◯ ◯ ◯ Conditions (3′) and (4′) ◯ ◯ ◯ Conditions (3″) and (4″) ◯◯ ◯ Lens arrangement of 1st lens unit: positive ◯ ◯ ◯ Lens arrangementof 1st lens unit: positive, negative ◯ ◯ ◯ Lens arrangement of 1st lensunit: positive, negative, positive ◯ ◯ ◯ 3rd lens unit virtual shapefactor 2.34 2.29 2.494 |(R3f + R3b)|/|(R3f − R3b)| Condition (5) ◯ ◯ ◯Condition (5′) ◯ ◯ ◯ Condition (5″) ◯ ◯ ◯ 3rd lens unit: at least twomeniscus lenses, each with a convex ◯ ◯ ◯ surface directed toward theobject side 3rd lens unit: at least three meniscus lenses, each with aconvex ◯ ◯ ◯ surface directed toward the object side Note: ◯ indicatesthat conditions are satisfied. X indicated that conditions are notsatisfied.

TABLE 3 7th embodiment 8th embodiment 9th embodiment Object-sidetelecentricity |En|/L (β = 0.3) 2.60 0.16 0.26 Object-sidetelecentricity |En|/L (β = 0.4) 34.46 0.37 0.41 Object-sidetelecentricity |En|/L (β = 0.5) 2.56 0.80 0.75 Image-side telecentricity|En|/|L/β| (β = 0.3) 0.23 0.19 0.27 Image-side telecentricity |En|/|L/β|(β = 0.4) 0.76 4.15 2.53 Image-side telecentricity |En|/|L/β| (β = 0.5)13.92 0.56 0.74 Conditions (1) and (2) ◯ ◯ ◯ Conditions (1′) and (2′) ◯X X Conditions (1″) and (2″) ◯ X X Difference in object-to-imagedistance between 0.3× and 0.5× 0.00000 0.00000 0.00000 Smallestobject-side F-number, MAXFNO 3.51 3.5 3.5 |ΔFNO/Δβ| 0.304 2.685 1.693Conditions (3) and (4) ◯ ◯ ◯ Conditions (3′) and (4′) ◯ ◯ ◯ Conditions(3″) and (4″) ◯ X X Lens arrangement of 1st lens unit: positive ◯ ◯ ◯Lens arrangement of 1st lens unit: positive, negative ◯ ◯ ◯ Lensarrangement of 1st lens unit: positive, negative, positive ◯ ◯ ◯ 3rdlens unit virtual shape factor 2.69 1.83 1.41 |(R3f + R3b)|/|(R3f −R3b)| Condition (5) ◯ ◯ ◯ Condition (5′) ◯ ◯ ◯ Condition (5″) ◯ X X 3rdlens unit: at least two meniscus lenses, each with a convex ◯ ◯ ◯surface directed toward the object side 3rd lens unit: at least threemeniscus lenses, each with a convex ◯ X X surface directed toward theobject side Note: ◯ indicates that conditions are satisfied. X indicatedthat conditions are not satisfied.

The image forming optical system of the present invention can be used inthe optical device, such as the motion picture film scanner (thetelecine device) or a height measuring device. An embodiment in thiscase is shown below.

FIG. 19 shows an example of the telecine device using the image formingoptical system of the present invention. This telecine device has alight source 11 for image projection on a motion picture film, a feedreel 12, a winding reel 13, a motion picture film 14 wound on thewinding reel 13 from the feed reel 12, an image forming optical system15 of the arrangement such as that shown in each embodiment of thepresent invention, and a CCD camera 16. Also in FIG. 19, a specificarrangement of the image forming optical system 15 is omitted.

In the telecine device, light emitted form the light source 11 isprojected on the motion picture film 14, and its transmission lightpasses through the image forming optical system 15 and is imaged by theCCD camera 16.

In the image forming optical system 15, the magnification can be changedso that the image information of the motion picture film 14 is acquiredover the entire imaging area of the CCD camera 16, to the size of themotion picture film, through the image forming optical system 15.

According to this telecine device, the image forming optical system 15has a bilateral telecentric design, so that even when the imagingmagnification is changed, the conjugate length remains unchanged.Consequently, there is no need to adjust the positions of individualmembers. Since the image-side F-number is maintained with littlefluctuation and the loss of the amount of light is kept to a minimum,the adjustment of brightness is not required. Furthermore, a change ofmagnification on the image plane caused by the disturbance of flatnessof the film that the object is photographed can be minimized.

FIG. 20 shows an example of a height measuring device using the imageforming optical system of the present invention. In this example, theimage forming optical system is used as a confocal optical system.

The height measuring device is constructed with a light source 21, apolarization beam splitter 22, a disk 23 provided with a plurality ofpinholes, a quarter-wave plate 24, a confocal optical system 25constructed in the same way as the image forming optical system in eachof the above embodiments of the present invention, an X-Y-Z stage 26, animaging lens 27, an image sensor 28, a motor 29 driving the disk 23, astage driving mechanism 30 driving the X-Y-Z stage 26, a sensor drivingmechanism 31 driving the image sensor 28, and a computer 32 controllingthe drives of the motor 29, the stage driving mechanism 30, and thesensor driving mechanism 31.

In the height measuring device constructed as mentioned above, alinearly polarized component p or s of light emitted from the lightsource 21 is reflected by the polarization beam splitter 22, passesthrough the pinhole of the disk 23, suffers a phase shift of 45° throughthe quarter-wave plate 24, and illuminates a point of a sample 33 on theX-Y-Z stage 26 through the confocal optical system 25. The lightreflected from the sample passes through the confocal optical system 25,suffers a phase shift of 45° through the quarter-wave plate 24, passesthrough the pinhole of the disk 23, is transmitted through thepolarization beam splitter 22, and is imaged by the image sensor 28through the imaging lens 27. The motor 29 is driven through the computer32, and thereby the entire surface of the sample 33 can be scanned. Inthis case, by finding the position where the intensity of light of aconfocal image of the sample 33 formed by the image sensor 28 ismaximized while changing the driving mechanism 30 or 31 in the directionof the optical axis, the height of the sample is detected. Moreover, themagnification of the confocal optical system 25 can be changed inaccordance with the size of the sample 33.

In the height measuring device also, the confocal optical system 25 hasa bilateral telecentric design, so that even when the imagingmagnification is changed, the conjugate length remains unchanged.Consequently, there is no need to adjust the position of individualmembers. Since the image-side F-number is maintained with littlefluctuation and the loss of the amount of light is kept to a minimum,the adjustment of brightness is not required.

What is claimed is:
 1. An image forming optical system comprising, inorder from an object side toward an image side: a first lens unit withpositive refracting power; a second lens unit with positive refractingpower; a third lens unit with negative refracting power; a fourth lensunit with positive refracting power; and an aperture stop interposedbetween the third lens unit and the fourth lens unit, a variablemagnification optical system being provided in which spacings betweenthe first lens unit and the second lens unit, between the second lensunit and the third lens unit, and between the third lens unit and thefourth lens unit are changed to thereby vary an imaging magnification,wherein the image forming optical system changes the imagingmagnification while constantly keeping an object-to-image distancethereof, and in at least one variable magnification state where theimaging magnification is changed, satisfies the following conditions:|En|/L>0.4 |Ex|/|L/β|>0.4 where En is a distance from a first lenssurface on the object side of the variable magnification optical systemto an entrance pupil of the image forming optical system, L is theobject-to-image distance of the image forming optical system, Ex is adistance from a last lens surface on the image side of the variablemagnification optical system to an exit pupil of the image formingoptical system, and β is a magnification of a whole of the image formingoptical system.
 2. An image forming optical system according to claim 1,further satisfying the following conditions: 1.0<MAXFNO<8.0 |ΔFNO/Δβ|<5where MAXFNO is an object-side F-number which is smallest when theimaging magnification of the image forming optical system is changed,ΔFNO is a difference between the object-side F-number at a minimummagnification of a whole of the image forming optical system and theobject-side F-number at a maximum magnification of a whole of the imageforming optical system, and Δβ is a difference between the minimummagnification of the whole of the image forming optical system and themaximum magnification of the whole of the image forming optical system.3. An image forming optical system according to claim 1, furthersatisfying the following condition: 0.6<|(R3f+R3b)/(R3f−R3b)|<5.0 whereR3f is a radius of curvature of a most object-side surface of the thirdlens unit and R3b is a radius of curvature of a most image-side surfaceof the third lens unit.
 4. An optical device using an image formingoptical system, the image forming optical system comprising, in orderfrom an object side toward an image side: a first lens unit withpositive refracting power; a second lens unit with positive refractingpower; a third lens unit with negative refracting power; a fourth lensunit with positive refracting power; and an aperture stop interposedbetween the third lens unit and the fourth lens unit, a variablemagnification optical system being provided in which spacings betweenthe first lens unit and the second lens unit, between the second lensunit and the third lens unit, and between the third lens unit and thefourth lens unit are changed to thereby vary an imaging magnification,wherein the image forming optical system changes the imagingmagnification while constantly keeping an object-to-image distancethereof, and in at least one variable magnification state where theimaging magnification is changed, satisfies the following conditions:|En|/L>0.4 |Ex|/|L/β|>0.4 where En is a distance from a first lenssurface on the object side of the variable magnification optical systemto an entrance pupil of the image forming optical system, L is theobject-to-image distance of the image forming optical system, Ex is adistance from a last lens surface on the image side of the variablemagnification optical system to an exit pupil of the image formingoptical system, and β is a magnification of a whole of the image formingoptical system.
 5. An image forming optical system according to claim 1,wherein a most object-side lens of the first lens unit has positiverefracting power.
 6. An image forming optical system according to claim1, wherein the first lens unit includes, in order from the object side,a positive lens, a negative lens, and a positive lens.
 7. An imageforming optical system according to claim 1, further satisfying thefollowing condition: 1.2<|(R3f+R3b)/(R3f−R3b)|<3.5 where R3f is a radiusof curvature of a most object-side surface of the third lens unit andR3b is a radius of curvature of a most image-side surface of the thirdlens unit.
 8. An image forming optical system according to claim 3,further satisfying the following condition:1.2<|(R3f+R3b)/(R3f−R3b)|<3.5.
 9. An image forming optical systemaccording to claim 5, satisfying the following condition:1.2<|(R3f+R3b)/(R3f−R3b)|<3.5 where R3f is a radius of curvature of amost object-side surface of the third lens unit and R3b is a radius ofcurvature of a most image-side surface of the third lens unit.
 10. Animage forming optical system according to claim 1, wherein the thirdlens unit has at least two meniscus lenses, each with a convex surfacedirected toward the object side.
 11. An image forming optical systemaccording to claim 3, wherein the third lens unit has at least twomeniscus lenses, each with a convex surface directed toward the objectside.
 12. An image forming optical system according to claim 5, whereinthe third lens unit has at least two meniscus lenses, each with a convexsurface directed toward the object side.
 13. An image forming opticalsystem according to claim 7, wherein the third lens unit has at leasttwo meniscus lenses, each with a convex surface directed toward theobject side.
 14. An optical device using an image forming opticalsystem, the image forming optical system comprising, in order from anobject side toward an image side: a first lens unit with positiverefracting power; a second lens unit with positive refracting power; athird lens unit with negative refracting power; a fourth lens unit withpositive refracting power; and an aperture stop interposed between thethird lens unit and the fourth lens unit, a variable magnificationoptical system being provided in which spacings between the first lensunit and the second lens unit, between the second lens unit and thethird lens unit, and between the third lens unit and the fourth lensunit are changed to thereby vary an imaging magnification, wherein theimage forming optical system changes the imaging magnification whileconstantly keeping an object-to-image distance thereof, and in at leastone variable magnification state where the imaging magnification ischanged, satisfies the following conditions: |En|/L>0.4 |Ex|/|L/β>0.4where En is a distance from a first lens surface on the object side ofthe variable magnification optical system to an entrance pupil of theimage forming optical system, L is the object-to-image distance of theimage forming optical system, Ex is a distance from a last lens surfaceon the image side of the variable magnification optical system to anexit pupil of the image forming optical system, and β is a magnificationof a whole of the image forming optical system, and wherein a mostobject-side lens of the first lens unit has positive refracting power.15. An optical device using an image forming optical system, the imageforming optical system comprising, in order from an object side towardan image side: a first lens unit with positive refracting power; asecond lens unit with positive refracting power; a third lens unit withnegative refracting power; a fourth lens unit with positive refractingpower; and an aperture stop interposed between the third lens unit andthe fourth lens unit, a variable magnification optical system beingprovided in which spacings between the first lens unit and the secondlens unit, between the second lens unit and the third lens unit, andbetween the third lens unit and the fourth lens unit are changed tothereby vary an imaging magnification, wherein the image forming opticalsystem changes the imaging magnification while constantly keeping anobject-to-image distance thereof, and in at least one variablemagnification state where the imaging magnification is changed,satisfies the following conditions: |En|/L>0.4 |Ex|/|L/β|>0.4 where Enis a distance from a first lens surface on the object side of thevariable magnification optical system to an entrance pupil of the imageforming optical system, L is the object-to-image distance of the imageforming optical system, Ex is a distance from a last lens surface on theimage side of the variable magnification optical system to an exit pupilof the image forming optical system, and β is a magnification of a wholeof the image forming optical system, and wherein the image formingoptical system further satisfies the following condition: 1.2<|(R3f+R3b)/(R3f−R3b)|<3.5 where R3f is a radius of curvature of amost object-side surface of the third lens unit and R3b is a radius ofcurvature of a most image-side surface of the third lens unit.
 16. Anoptical device using an image forming optical system, the image formingoptical system comprising, in order from an object side toward an imageside: a first lens unit with positive refracting power; a second lensunit with positive refracting power; a third lens unit with negativerefracting power; a fourth lens unit with positive refracting power; andan aperture stop interposed between the third lens unit and the fourthlens unit, a variable magnification optical system being provided inwhich spacings between the first lens unit and the second lens unit,between the second lens unit and the third lens unit, and between thethird lens unit and the fourth lens unit are changed to thereby vary animaging magnification, wherein the image forming optical system changesthe imaging magnification while constantly keeping an object-to-imagedistance thereof, and in at least one variable magnification state wherethe imaging magnification is changed, satisfies the followingconditions: |En|/L>0.4 |Ex|/|L/β|>0.4 where En is a distance from afirst lens surface on the object side of the variable magnificationoptical system to an entrance pupil of the image forming optical system,L is the object-to-image distance of the image forming optical system,Ex is a distance from a last lens surface on the image side of thevariable magnification optical system to an exit pupil of the imageforming optical system, and β is a magnification of a whole of the imageforming optical system, and wherein the third lens unit has at least twomeniscus lenses, each with a convex surface directed toward the objectside.
 17. An optical device using an image forming optical system, theimage forming optical system comprising, in order from an object sidetoward an image side: a first lens unit with positive refracting power;a second lens unit with positive refracting power; a third lens unitwith negative refracting power; a fourth lens unit with positiverefracting power; and an aperture stop interposed between the third lensunit and the fourth lens unit, a variable magnification optical systembeing provided in which spacings between the first lens unit and thesecond lens unit, between the second lens unit and the third lens unit,and between the third lens unit and the fourth lens unit are changed tothereby vary an imaging magnification, wherein the image forming opticalsystem changes the imaging magnification while constantly keeping anobject-to-image distance thereof, and in at least one variablemagnification state where the imaging magnification is changed,satisfies the following conditions: |En|/L>0.4 |Ex|/|L/β|>0.4 where Enis a distance from a first lens surface on the object side of thevariable magnification optical system to an entrance pupil of the imageforming optical system, L is the object-to-image distance of the imageforming optical system, Ex is a distance from a last lens surface on theimage side of the variable magnification optical system to an exit pupilof the image forming optical system, and β is a magnification of a wholeof the image forming optical system, and wherein the image formingoptical system further satisfies the following condition:1.2<|(R3f+R3b)/(R3f−R3b)|<3.5 where R3f is a radius of curvature of amost object-side surface of the third lens unit and R3b is a radius ofcurvature of a most image-side surface of the third lens unit, and thethird lens unit has at least two meniscus lenses, each with a convexsurface directed toward the object side.